Instructions
Turn on your Xbox 360 console and wait for the dashboard to load.
Go to the "My Xbox" portion of the dashboard and scroll across one space to your gamertag. Press the "A" button.
Select "Profile", then "Edit Profile", and choose "Gamertag".
Choose the "Enter New Gamertag" option.
Type your new gamertag into the text field and press "Done".
Confirm that you wish to change your gamertag and pay 800 Microsoft Points to do so.