So, I have the Flora Early Join file on my computer for Fire Emblem Fates, and I have no experience on downloading 3DS game mods/patches. My 2DS XL already has PicHaxx.
First off follow the 3ds.hacks.guide to put luma on your 3ds if you haven’t so already.
Next go here: https://gist.github.com/PixelSergey/5dbb4a9b90d290736353fa58e4fcbb42
This will tell you how to patch games. But only if you have Luma! I am not familiar with patching games without it!