Okay, so this one's a bit tricky.
Beat the game in single-player mode to get Bowser out of the way. Then when you go to select a file, you can press L and R buttons as you select a file, by pressing A.
Phew! If you've followed that, the next time you begin playing, you'll find yourself controlling Luigi.

Super Mario Bros 2 3DS Cheats
Want to play as Luigi? Here's how!
Comment Now!