Check the ROM hacking forum and/or the tutorial forum and you will find how to open a 3ds rom, extract its datas and edit them (example: change the text) then recompile the rom together.
Right now, there are two ways to patch a game:
- you open, extract, modify, recompress the rom and play it on your console through different means,
- instead of remaking the entire rom file, you keep the edited part and place it on your SD card, then run the real game on your console through HANS (homebrew) after activating the "on-the-fly" patch (it will replace the game datas with the ones you prepared)