There's really no reason to remove custom firmware. Your cousin or you can play the 2DS like it is stock. Just use it normally like it is without messing with anything, cheating, modding, or playing homebrew.
But if you intend to go back to stock firmware at all cost, you have to update to boot9strap first.
Once you've completed that, you can uninstall with this:
But if you intend to go back to stock firmware at all cost, you have to update to boot9strap first.
Once you've completed that, you can uninstall with this: