Where do you change your pokemon's moves?

Well if you want to delete your pokemons moves you can go to the move deleters house. That's in Jubilife or Hearthome city I think? If you want to teach it new moves you can get a heart scale and go to Pastoria city. In one of the houses, there is a guy who will teach your Pokemon moves for a heart scale. You can get heart scales under ground.

at the move deleters place or the move teacher in the game