The question should more sensibly be "why?", but here we are:
In the top right of the page, click “Control Panel”. On the new page, under “My Settings” on the left hand side, click “Edit Profile”. Then type in your new username in the box underneath the words “Username Change - Optional” and scroll down to the bottom and click “Save Changes”.
Voila: You are now officially an unstable ***** who can’t make his mind up or stick to a plan.