At the main menu, press X, L, X, L, X, L(2), R to unlock level select and all challenges. Note: There are two side effects after using the level select cheat. First, stealth attacks no longer give you dog tags, which means you cannot advance to a higher strike level. Get all the tags you need before using the cheat. Secondly, when you save a game in which the level select code is used, the file is now marked with an "X" just before the time and date.