Its one of three options after hitting a prestige in the prestige menu. It will reset you back to private level 1.
Oh, cool. So I don't even need a prestige token to do this? (on Prestige 2)
You do. Once you prestige it gives you three options to either reset your stats, pick an extra class, or refund all of your unlock tokens. You can only pick one per prestige. So if you are on prestige 2 you have to prestige to 3 and then pick reset your stats and you will revert back to level 1 no prestige. Hope this helps!