This is a feature that I keep thinking I have written and yet apparently I haven't.
If you want to change the actual user account, you can just run the real postgres command:
alter user yourusername password 'yournewpassword';
This will not currently change the password used by the training wheels system and tutorial pages making it now slightly annoying. Let me take a look to see if I can quickly get this part working too though. If you have completely lost your password then come chat in IRC to recover the account.