I don't see an obvious undo button, but upon switching to a keyboard with ctrl (I'm on mobile currently) ctrl-Z does it.
Also, the undo only works within editors. It doesn't work across editors. It takes some time getting used to, but I think this way is actually better. I was frequently alternating between taking notes & writing the chapter, & I didn't want to have to undo the work on my chapter in the process of undoing the work on my notes. So I just took out the global undo. It reduced LoC by alot & improved UX.