Being able to change your email address is a known gap, sorry about that. For now if you want to use a different email you'll need to make a second account. We plan to make account merging possible in the future.
A short-sighted technical decision on my part: we use the Firebase Auth UIDs as our internal user IDs, but Firebase Auth does not allow you to have multiple email auth-providers on a single account. So we need to add a layer of indirection in our data model.
Thank you! The inability to change the email address is OK, but it should still appear somewhere so I know what it is and that it's not my username. Maybe a note of "you can't change this yet, sorry!" would be nice too.
Also, having a section in the profile where I could change my login email address would help with the UX.