
Yeah I'll do a backup but does replacing the boot.firm file reset my data or something?Before doing stuff like this, I suggest making a NAND Backup: https://3ds.hacks.guide/godmode9-usage.html
These backups completely save a 3DS's current state, so that if something doesn't go as planned, you could always restore the backup, and everything will be as it was.

The Boot.firm overrides the 3DS's boot, so that it would boot into a CFW or such instead of Nintendo's firmware. It will not delete your data to replace it.Yeah I'll do a backup but does replacing the boot.firm file reset my data or something?