Go to Edit Profile --> there will be a textbox that will be labeled Username. All you have to do is type in a new username for yourself, enter your password, and save it. I believe you can only change usernames either twice, or three times and then you have to pay 10k.
Anyway, this really isn't the right place for this question. Next time you have an inquiry of the sort, go to Questions & Assistance. sweatdrop