Instructions
Power on your Xbox 360 console, and then insert the "Modern Warfare 2" disc into the disc drive. The game will start automatically once the console reads the disc.
Press the "Start" button on your controller, and then sign into your Xbox Live Gamertag account.
Scroll to "Multiplayer" and then press "A" to select it.
Press "A" to select Xbox Live.
Scroll to "Barracks" and then press "A" to select it.
Scroll to "Prestige Mode" and then press "A" to select it.
Follow the prompts to confirm that you wish to reset your rank to Level 1. Once you have entered Prestige Mode, you cannot undo the action.