Newest firmware needs a downgrade. Two solutions exist:
1) Hardware modding - requires a ton of experience with micro soldering (eg. I don't trust myself doing it), and a lot of tools that you may or may not have. You can find a local computer guy/gal to do it (I used to have a connection at a store that did actual installation of modchips and the like BUT they are like 3 hours away by transit) OR there is a list here by region.
2) Software modding - requires a second 3DS/2DS running custom firmware already. Basically you buy a legit game with a throwaway NNID - Fieldrunners OR Legends of Exidia, and inject the save data on the custom firmware one. You then do a system transfer and transfer the game with the modified save data onto the stock 3DS. You copy files for the NFIRM downgrade (this writes code to the chip, essentially overwriting the system files that prevent you from downgrading), and you open Fieldrunners or Legends of Exidia on your system. And then you proceed with the "normal" tools for downgrading.