However it is installed from their GitHub repo. So in theory you should be able to checkout the v9 branch. I'm guessing that there may need to be changes in the conf file (I think /etc/odoo/) and perhaps some DB updates too? I'm only guessing though really. I would expect that upstream have instructions on how to upgrade from v8 to v9. Keep in mind that TurnKey v14.x is Debian Jessie under the hood and hopefully it should be fairly straight forward.

If you have any success it'd be great if you can share your info as it may help out others.