Using NSPs (backups) of Smash Ultimate carries specific risks because Smash is an online-heavy game.
Assuming you have a (running Atmosphere or SXOS) and have already backed up your NAND, follow this guide. Super Smash Bros. Ultimate Switch NSP UPDATE DLC
(Xenoblade Chronicles 2) – Includes Cloud Sea of Alrest stage. Kazuya Mishima (Tekken) – Includes Mishima Dojo stage. Sora (Kingdom Hearts) – Includes Hollow Bastion stage. Bonus Content Piranha Plant : The standalone early-purchase bonus fighter. Using NSPs (backups) of Smash Ultimate carries specific
Supplemental NSP files that include essential game data, balance adjustments, and bug fixes. Most DLC character data is actually included in these free updates; the specific DLC file often acts only as an "unlocker". Kazuya Mishima (Tekken) – Includes Mishima Dojo stage
Which specific are you using to manage your library?
DLC NSP files unlock the massive waves of content released across two individual Fighter Passes. This includes legendary gaming icons that do not exist in the base cartridge data: