Hi there. Assuming that you are using TurnKey v15.1 or v15.2, then you should be able to follow the instructions as noted in the Official ownCloud docs.

If you aren't sure of your TurnKey version and wish to check, then please run the following command:

turnkey-version

If you're using TurnKey v15.2 (turnkey-owncloud-15.2-stretch-amd64) then you will have PHP7.2. If TurnKey v15.1 (turnkey-owncloud-15.1-stretch-amd64) then you'll only have PHP7.0. However, my reading of their system requirements docs suggests that it should still work with PHP7.0 (although future ownCloud versions may require newer version of PHP).

I hope that helps. If you get stuck, or hit any troubles, please post back. Actually, it'd be awesome if you post back regardless, to let us know how you get on and any pointers that might help other users upgrading.