You can change your username from your settings; you must have 1000 Stars, or 500 Stars if you have Starpass.

To change your username on-site, you should do the following:

1. Click on the drop down menu next to your avatar headshot
2. Click on your “Settings” tab
3. Go to the “Account” tab
4. Type in a new username in the “Avatar Name” box
5. Click “Submit”

If the username is not available, you will receive a notice beforehand. However, if your username change goes through, you will be notified and your username will change on-site immediately.