apt-get upgrade will then install any updates. As noted, TurnKey installs all security updates nightly, so the only updates you'll get will be non-security related ones.

Generally these updated packages will be fine. However, they aren't always as well tested as security updates, so on odd occasions there have been regressions in non-security updates. Having said that, in my experience, that is quite rare. And worst case scenario, you can re-install the previous version if need be. My last point is covered in some more detail in this Ask Ubuntu answer.

FWIW in case you weren't aware, Ubuntu is also based on Debian, so often general Ubuntu instructions also apply to Debian. Having said that, unlike TurnKey, Ubuntu is NOT binary compatible with Debian. So installing specific software on Debian which was built for Ubuntu (e.g. using Ubuntu PPA repos) is a bad plan! It may work initially, but it's likely a matter of time before it bites you!

PS this info is generic and applies to all TurnKey appliances.