I'm assuming this is for Pokemon? Get save manager on your 3ds (no cfw needed use soundhax) and get PKhex on your PC. Back up your Pokemon save file, and open it on your PC with pkhex. It should be in the folder saveDataBackup. Once you're done playing with pkhex export it as a .Sav and replace the file in saveDataBackup with that file. Also if I were you I'd back up your original save. Also, save manager works on games other than pokemon, so keep that in mind.