You have several options:
- Check the admin menu for the “Update software” entry. This is the easiest way.
- From webconf => software => update
- From command line:
update_zynthian.sh
After this, for changing the software version you are using:
- stable => default
- staging => normal testers => quite stable.
- testing => advanced testers => not so stable!
- custom (advanced users only!)
You should access webconf → software => repositories
Enjoy!