Version 12 was released yesterday and Bare Bones Software published info on how to upgrade, depending on which version you’re on — you can find it here. It’s probably easiest to just upgrade when they ask you to — completely worth it in my opinion. They also published a complete change log:
BBEdit 12.0 contains many new features, enhancements, and refinements to existing features. It also includes fixes for reported issues. This document describes changes in BBEdit since the previous update (11.6.8).
BBEdit also has a wonderful manual which is totally worth reading. It’s probably the first software manual that I have read in my life.