https://3ds.hacks.guide/ntrboot
I haven't done it myself, but I think the actual ntrboot flasher part should be fairly simple assuming your card is supported, just follow from that page of the guide through to dumping your flash, there is no need to actually flash ntrboot onto your flashcard so you can skip anything besides the backup dumping.
Once you've got the backup, editing it isn't exactly noob friendly but it's not
too bad. Keep in mind that I haven't done this myself so I can't guarantee it'll work, but I am familiar with editing NDS banners in general and I know it's been done before so it'll probably work.
- Backup your flashcard using ntrboot flasher
- Download ntrbootbanner.py (right click -> save as)
- If you don't have Python 3, install it
- It's a very simple script, the NDS file starts at 0x2000 in the backup bin, banner offset is a u32 at 0x68 in the NDS header, banner is probably 0x840 bytes at 0x2000 + banner offset, just made this little script so I don't have to explain that if you're not familiar with hex editing, if you are feel free to do that manually instead
- In the terminal run
python3 ntrbootbanner.py -x banner.bin path/to/backup.bin
(replace path/to/backup.bin with your actual path, you can usually just drag drop the file onto the terminal/cmd window)
- You now should have a banner.bin file that you can edit using any DS banner editor such as MessAnimDSIco or create a new one using dsibanner.py (right click -> save as), NDS Banner Editor may work too, however it seems to always export a DSi type banner which will most likely not work in this case
- Once you have your new/edited banner.bin, you can inject it back to the backup bin with
python3 ntrbootbanner.py -i banner.bin path/to/backup.bin
(again, replace path/to/banner.bin with the real path as above)
- Restore your edited backup bin using ntrboot flasher (this part of the guide)
Please let me know if this doesn't work and I'll see what I can do about it, I know for certain this is possible for some cards at least as I've seen it done, however I don't remember it being explained how it was done and I've never done it myself it's possible there's more that needs to be done then just edit the banner. There might be some kinda hash in those first 0x2000 bytes before the NDS file starts or something, not sure what those are...