Currently the only way to evolve Meltan is through Pokemon Go with 400 candies.
Thank you to Boaty McBoatface for bringing this back to attention.
You have to bring in an already-evolved Melmetal.
There is no way to obtain Meltan in-game in SwSh, so the only way to obtain it or Melmetal is through Pokemon HOME; transferring from LGPE (or Go if that is possible but probably not) once HOME is released. Therefore, we do not know (unless someone has hacked in a Meltan).