How do I change my Facebook username?

You can only change the username for your personal timeline once. To change your username:

1. Click at the top right of any Facebook page and choose Account Settings
2. Click General in the left-hand column
3. Look for the setting Username and click Edit to the far right
4. Type your new username in the open field and click Save Change
5.  

Ref: http://www.facebook.com/help/