You normally can't. Only if you have a very good reason or if you are a VIP user, then you can ask theymos and only if he accepts, he will change your username.
A newbie here, dont know yet whos that theymos you mean... Hahahah
But I will explore:)
Thank you so much for the Info:)
is the forum admin.
VIP is a user rank which costs 50 BTC.
So in your case, it's probably better if you just create a new account.