Press start during the game and while on the pause menu hold the back button, Left Trigger, and Right trigger and enter the password Y, RB, Y, RB, X.

From the single player Campaign Menu, go to the "Missions" screen and while holding BACK + LT + RT input YELLOW, RB, YELLOW, RB, BLUE. A tone should confirm that you did it right and you should now be able to choose any mission you want.

Submitted by DayDay
Correction by incdecline