Unless you already own a game card that can update your system to a firmware at or over 9.2 firmware, you have no choice but to buy a game card (or sky3ds flashcart)
OoT and CN directly to 9.2 are your safest choices, but if you update past 9.2, you are risking having only one chance to successfully downgrade unless a better downgrading method comes out, which I don't think will happen anytime soon.