Uh idk ? Try going to your profile then click the pen thingy by your name (if you haven’t tried it already). You might be too new but I doubt it
Contact support by sending a help ticket. You can change your username using your *first* free name change and after that you gotta use 20 extra credits to do so.
If you have done as previous players have said (gone to profile and pressed the little edit button)and not used your extra credits...Maybe your "new username" is taken already? Also wanna remind you there's no need to send ATA a help ticket anymore to free up a name...they made the system automatically check that when you try to change name. But if you're still struggling, you can of course always send in a help ticket anyway and try get help! Locking now.