Instructions
Turn on your Xbox 360, and choose ̶0;My Xbox.̶1;
Select your avatar, and press the ̶0;A̶1; button on the controller.
Choose ̶0;Profile̶1; from the menu, and then select ̶0;Edit Profile.̶1;
Choose ̶0;Gamertag̶1; from the menu. You will see information about changing your gamertag.
Press ̶0;Continue̶1; to confirm that you would like to change your gamertag.
Use the on-screen keyboard to type in your new gamertag, and press ̶0;Done.̶1; The system will alert you if the gamertag you want is already in use. If it is, you will have to choose another.
Press ̶0;Confirm̶1; to finalize the change, which will deduct 800 Microsoft points from your account.