Before you can turn into a Legend you need to level up through the different levels in multiplayer until you hit 50. Once you hit level 50 you will be given the change to start over again as a Legend.
To change your status from rank 50 back to a rank 1 Legend hit while in multiplayer, then go down to the multiplayer tab. You should see your multiplayer profile on the screen; press while on this screen and you will pass through to your next round of leveling. Just be careful not to hit while in this menu if you don't want to level up just yet.
If you have passed over and already become a Legend, this achievement will unlock automatically once you start playing multiplayer.