Instructions
Modding the Service Tag
Boot up the game and wait for the main menu to pop up.
Press "Start" to bring up the settings box and then scroll to the right to reach the "Settings" tab.
Scroll down to and select "Appearance."
Select "Service Tag" to bring up an on-screen keyboard.
Enter the desired service tag and then press "Start." Press "B" to exit the settings screen.
Changing your Gamertag in General
Go to the Xbox dashboard and scroll up to "My Xbox."
Scroll to the right until you reach your gamer profile and select it.
Scroll down to "Change Gamertag" and select it. A note will pop up indicating that changing your gamertag costs 800 Microsoft Points. You will be prompted to add more points via credit card if you do not have enough in your current balance.
Select "Enter New Gamertag" then create one with the on-screen keyboard. Select "Yes, Use This Gamertag" to confirm your selection.