If you are using a 3DS with Custom Firmware (CFW) like Luma3DS, you can install the update as a .cia file using the application. How to Update Pokémon Ultra Sun and Pokémon Ultra Moon

While the update is technically a bug fix, its primary role was to stabilize high-level competitive and online play.

While this update was originally required for online play, official Nintendo Network services for the 3DS family ended on April 8, 2024. You can still use the update for local play or potentially for community-run revival servers. How to Update Pokémon Ultra Sun and Pokémon Ultra Moon