https://github.com/Plailect/Guide/wi...ware-Downgrade
For 11 it appears you have to downgrade via hardmod or DSiWare and second 3DS with CFW.
Edit:
If the OP is only after saves backup/editing (and wants to stay 100% legit, no flashcard or hacks) I would research something like NDS Adaptor Plus. I'm pretty sure it can be used to wipe saves from 3DS carts like Resident Evil The Mercenaries 3D that otherwise can't be erased, but I have no idea about how compatible it is with other (and more recent) games.