Search for "Pokémon Ultra Sun Update" or find it in the "Settings/Other" -> "Updates" section. Select the update and download it.
Copy the downloaded Pokemon_Ultra_Sun_Update_1.2_WORLD.cia file into a folder on your SD card. It is highly recommended to create a folder named cias in the root directory to keep things organized. Pokemon Ultra Sun- Update 1.2 3DS -WORLD- CIA -...
While "WORLD" or "Region-Free" CIAs are highly versatile, double-check that your base game version matches the update. If you have a European base game, a strictly USA-coded update might create a separate game title ID instead of patching your existing game. A true "WORLD" CIA circumvents this problem. Search for "Pokémon Ultra Sun Update" or find