So at the commandline of your appliance (either via SSH or Webshell) type:

apt-get update && apt-get install owncloud

It should say that it will download a significant amount of files (mine wants to download 164MB!) so just hit 'y' and let it run through... If for some reason it doesn't do that you could try:

apt-get install owncloud=6.0.2-1

Please note that these instructions will date quickly and once there is a new release the last bit ('=6.0.2-1') will need to correspond to the full new version number. That can be discovered with this command:

apt-cache showpkg owncloud

apt-get update will update the available package lists which should (hopefully) allow you to update.

However, the ownCloud package hasn't always been available and if you have a very old TurnKey ownCloud appliance it may have been prior to ownCloud being installed by package management. If so I suggest that you migrate your data to a new TKL ownCloud instance and then update ownCloud to the latest.

I tried upgrading from version 7.0.2 to 8.0 using 'apt-get install owncloud-server' but it was not a clean upgrade.  Somehow the file '/var/www/owncloud/.htaccess' was removed but not replaced with the correct version, and the new file ownership and permissions were incorrect.

I had to download the version 8.0 .htaccess file from https://github.com/owncloud/core.  Next I copied a script from https://doc.owncloud.org/server/8.0/admin_manual/installation/installation_wizard.html#setting-strong-directory-permissions.  The script had to be modified as follows to correctly handle all the .htaccess files.  I called it fix-perms.

#!/bin/bash
ocpath='/var/www/owncloud'
htuser='www-data'

find ${ocpath}/ -type f -print0 | xargs -0 chmod 0640
find ${ocpath}/ -type d -print0 | xargs -0 chmod 0750

chown -R root:${htuser} ${ocpath}/
chown -R ${htuser}:${htuser} ${ocpath}/apps/
chown -R ${htuser}:${htuser} ${ocpath}/config/
chown -R ${htuser}:${htuser} ${ocpath}/data/

chown root:${htuser} ${ocpath}/.htaccess
chown root:${htuser} ${ocpath}/data/.htaccess
chown root:${htuser} ${ocpath}/config/.htaccess

chmod 0644 ${ocpath}/.htaccess
chmod 0644 ${ocpath}/data/.htaccess
chmod 0644 ${ocpath}/config/.htaccess

After that, I was able to login and run the database update.  Fortunately, the data directory and config.php files were not disturbed.  So far it looks like everything is working okay.  I'm using owncloud-client ver 1.7.1 on Ubuntu 14.04 LTS.

I received a reply from the ownCloud package maintainer telling me, that I should not be using apt-get install owncloud-server.  I started over (thank God for snapshots) and tried apt-get update && apt-get upgrade && apt-get install owncloud once more, but still no joy.  I tried cleaning the apt cache and running apt-get install -f owncloud but still no luck.  I checked the repository and found that both old and new versions were present.

# apt-cache show -a owncloud
Package: owncloud
Version: 8.0.2-8
Architecture: all
Maintainer: Jürgen Weigert <jw@owncloud.com>
Installed-Size: 49
Depends: owncloud-server-core, owncloud-config-apache
Recommends: mysql-server, curl, exim4 | mail-transport-agent
Suggests: clamav | clamav-daemon, smbclient, libreoffice-writer
Conflicts: owncloud-fhs
Provides: owncloud
Filename: ./all/owncloud_8.0.2-8_all.deb
Size: 26922
MD5sum: fb03273b2431f01d4cb2e462f346d61a
SHA1: eb0d9d6c8eb293d88666e4e3e087ea3413d0fa51
SHA256: b67f6ae57a9d3d6e116975a4c1f9081f9110c62d387ac5a02292a116a9aae8af
Section: devel
Priority: optional
Description: ownCloud Server - Private file sync and share server
 ownCloud Server provides you a private file sync and share
 cloud. Host this server to easily sync business or private documents
 across all your devices, and share those documents with other users of
 your ownCloud server on their devices.
 .
 ownCloud - Your Cloud, Your Data, Your Way!  www.owncloud.org

Package: owncloud
Status: install ok installed
Priority: optional
Section: devel
Installed-Size: 103611
Maintainer: Jürgen Weigert <jw@owncloud.com>
Architecture: all
Version: 7.0.2-1
Depends: apache2 | httpd, php5 (>= 5.3.3), php5-mysqlnd | php5-mysql, php5-sqlite, php5-pgsql, php5-common, php5-gd, php-xml-parser, php5-intl, php5-mcrypt, php5-curl, php5-json, ntp | time-daemon
Recommends: curl, exim4 | mail-transport-agent, php5-imagick, php5-apc
Suggests: clamav | clamav-daemon, smbclient, libav-tools | ffmpeg, libreoffice-writer
Conffiles:
 /etc/apache2/conf-available/owncloud.conf 8e2ddaaca00711d181fc467c11fbaad9
 /etc/apache2/conf.d/owncloud.conf 8e2ddaaca00711d181fc467c11fbaad9
Description: ownCloud Server - Private file sync and share server
 ownCloud Server provides you a private file sync and share
 cloud. Host this server to easily sync business or private documents
 across all your devices, and share those documents with other users of
 your ownCloud server on their devices.
 .
 ownCloud - Your Cloud, Your Data, Your Way!  www.owncloud.org

On a hunch, I tried specifying the version as described previously in this thread.

# apt-get install owncloud=8.0.2-8
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  owncloud-3rdparty owncloud-app-activity owncloud-app-external
  owncloud-app-files owncloud-app-files-encryption owncloud-app-files-external
  owncloud-app-files-locking owncloud-app-files-pdfviewer
  owncloud-app-files-sharing owncloud-app-files-texteditor
  owncloud-app-files-trashbin owncloud-app-files-versions
  owncloud-app-files-videoviewer owncloud-app-firstrunwizard
  owncloud-app-gallery owncloud-app-provisioning-api
  owncloud-app-templateeditor owncloud-app-user-external
  owncloud-app-user-ldap owncloud-app-user-webdavauth owncloud-config-apache
  owncloud-server
Suggested packages:
  clamav clamav-daemon smbclient php5-imagick libav-tools ffmpeg php5-ldap
  php5-apcu php-apc
The following NEW packages will be installed:
  owncloud-3rdparty owncloud-app-activity owncloud-app-external
  owncloud-app-files owncloud-app-files-encryption owncloud-app-files-external
  owncloud-app-files-locking owncloud-app-files-pdfviewer
  owncloud-app-files-sharing owncloud-app-files-texteditor
  owncloud-app-files-trashbin owncloud-app-files-versions
  owncloud-app-files-videoviewer owncloud-app-firstrunwizard
  owncloud-app-gallery owncloud-app-provisioning-api
  owncloud-app-templateeditor owncloud-app-user-external
  owncloud-app-user-ldap owncloud-app-user-webdavauth owncloud-config-apache
  owncloud-server
The following packages will be upgraded:
  owncloud
1 upgraded, 22 newly installed, 0 to remove and 0 not upgraded.
Need to get 23.6 MB of archives.
After this operation, 34.7 MB disk space will be freed.
Do you want to continue [Y/n]?

As you can see, it not only installed the missing owncloud-config-apache pkg, but also several other packages that were not installed when I ran apt-get install owncloud-server.  Sorry for any confusion I may have caused.  I had to remove the old apache config file for owncloud and enable UTF8 in php.ini.

Make sure that you have a good backup of your server (just in case something goes wrong) then away you go!

Mark,  See my reply to Peter below for an update strategy.

I started to reply to your message once before, but an errant click about half-way through caused me to lose the entire post.  Haven't had time until now to try again.

As I said below, I haven't tested the entire upgrade process, but the later portions worked well.  I hope the strategy is helpful.

Peter, While it appears the older versions are no longer available, the last release of each major version is still available on OpenSUSE's Open Build Service.  The key is to remember that each major version is maintained in a separate repository. The ownCloud changelog contains links to the older versions. For example the 5.0.19 release can be found by following the link Packages: on the Open Build Service. Click on the Debian icon to get the instructions for the Debian repositories.  For Wheezy (Debian 7) use the following:

Note: Add the OpenSUSE Release.key before trying to run the update.  This step should only be required once (same key for all repositories)

echo 'deb http://download.opensuse.org/repositories/isv:/ownCloud:/community:/5.0/Debian_7.0/ /' >> /etc/apt/sources.list.d/owncloud.list 
apt-get update
apt-get install owncloud

wget http://download.opensuse.org/repositories/isv:ownCloud:community:5.0/Debian_7.0/Release.key
apt-key add - < Release.key  

Also we hope to not let the time between releases stretch out this far again...

@John, thanks so much for helping out! :)

Although we have not decided whether we will go with ownCloud 7 from the Debian repos (which will stay at version 7 but will get daily auto security patches so will automatically be secure). Or go with upstream ownCloud (which won't get auto security updates but will be easier for end users to manually update).

At this point we are leaning towards the ownCloud 7 (from Debian repos) direction and documenting how to update for those that want that.

Out of interest what would your preference be and why

And I haven't been true to my stated plan (in a post above) to provide copy/paste type documentation for updating to upstreams version... Apologies on that; but I'm not clear on when I will get a chance to do that...

Regardless I have created an issue/feature request on our issue tracker so it doesn't get forgotten (see here). If anyone wants to have a crack at this before I get to it, that would be warmly welcomed! :)