For those using homebrew or emulators like Citra, installing the update via a .cia file is the standard method.
Pokémon Ultra Moon Version 1.2 update was primarily a maintenance patch released in early February 2018 to resolve critical game-breaking bugs that affected competitive play. While it introduced no new content, it was mandatory for accessing all online features, such as the Festival Plaza and Battle Spot. PocketMonsters.net Core Changes and Bug Fixes pokemon ultra moon 1.2 cia