Instructions
Start your Xbox 360 and choose the "My Xbox" option by using the directional pad and pressing the "A" button.
Choose the "Profile" option from the menu that comes up. This option will be at the top of the menu.
Select "Edit Profile" from the options list. It will be at the top of the list.
Select "Gamertag" from the next list of options. It will be at the top of the list.
Read the gamertag information and choose the continue button. A box will pop up that will let you type in a new gamertag. To choose the numbers and letters for your new name, use the directional pad and the "A" button.
Select the "Done" button on the screen when you are finished typing the new name.
Select "Confirm" when the confirm screen pops up. This will spend 800 gamer points, so make sure that you want to change your name to the new name you just entered. When you start up COD 5 you will see that your name has changed.