You go to Contact the Staff, and find the “Change Username” item and tell them what you want your username to be.
Keep in mind that it takes a while, like a few days, to get your username change. Sometimes not even a full hour. It depends on how busy the staff are.
There’a a sub-forum of the Bell Tree HQ called “Contact the Staff”. Over there, create a thread, and find the Username Change from the drop-down menu under “Category”.