You can’t do it now
Hyrule Warriors Legends came out last week on 3DS in Japan, and people are already clamoring for a pretty basic feature — save data reset. Yep, if you just want to replay the game from a fresh file or resell your game with no data on it, you currently cannot do that in the present edition of Legends.
It looks like the developers have caught this oversight though, and are responding to fan demand, as it will be added at a later date by way of a patch. While this doesn’t seem like that big of a deal for the west as we still aren’t getting it until March, inherently, this update will likely be present in the localized edition.