You have to mail [email protected]
to do this. There is no simple way to do this and often times the developers won't do it. As far as I know, there's just been one person who has changed their avatar name in the past. And there have been other people who wanted to change their names but it didn't happen for them for whatever reason (one even being a guide).
So for changing your user name, possibly. But for being able to change it back, almost certainly no.