Instructions
Press "Start" on your Xbox 360 controller at any time during an online or offline game of "Call of Duty: Modern Warfare 2." This will pause the game and take you to the menu. Note that in online games this will only bring up the menu, leaving you vulnerable to attacks from enemies.
Select "Options" from the menu.
Open the "Controller Settings" menu item.
Scroll through the list on screen and select a new layout with which you feel comfortable. As you highlight each layout, the button controls for that layout will appear on screen. For example, choosing the "Tactical" layout allows for faster shooting via a "Drop Shot" that is performed using the right analog stick.
Select the layout you want to use. Your Xbox 360 controller will be modified to that layout. Press "Start" to resume the game.