The most common reason a Pokémon Ultra Moon update fails to apply (even after a successful installation message in FBI) is a region mismatch.
: Resolved an issue where the move Ion Deluge failed to function correctly.
The 3DS titles updates sequentially. If you have never updated, you are on v1.0. This is the 12th revision released, hence "Update 12.cia."
: Certain CFW settings, specifically "Enable Game Patching," can ironically cause the 1.2 update to crash on some systems. Step-by-Step Fixes
[SD Card Root] └── [cia] └── Pokemon_Ultra_Moon_Update_v1.2.cia <-- Install via FBI Troubleshooting Common Errors