We haven't documented this properly but currently you can change your username once yourself, after that, an admin has to do it for you. This is to prevent abuse.
On mobile I can edit the display name field under edit account, but I haven't ever changed my username, so I don't know if it goes unmutable after changing it once.
I suggest messaging the moderators via Intercom. They will probably change your username if you don't do it too often.
I could swear there used to be an option for changing one's username (I've done it before). Has this option been removed? Am I just too daft to find where to click? Or is it auto-disabled after you've done it once?