Well for ReiNX, the guide is located
here but the build for 6.2 support can be found
here.
You can only unlink the account if you format the console as explained
on their website but its better to have one linked than not to have one at all because some applications need a linked account to work such as the NES app or the game Pianista. Plus Nintendo primarily bans consoles, not accounts.
You don't need to update your firmware, or at least SysNAND, to the latest to use EmuNAND. And as of SX OS V2.4, they support firmware 6.2 which is the latest. As for a guide,
here is one found on GBATemp.