How do I change my username (No, I am not hiding)

[inernetster]

I clicked on username to purchase for something like 16 dnf$ and it said purchase completed. Now, how do I change my name.. I thought there would be a box for the new name.
Any help would be appreciated
thx

note 4/28: I must have clicked the wrong thing. Went back in and saw for 2500 dnf$ to do the new username and there was a box to input it into .. just have to buy a feww dnf$ soon