How do you get the different forms of Deoxys?

Answer 1

In Pokemon FireRed, LeafGreen, Ruby, Sapphire, and Emerald, you cannot change Deoxys' forme. Each game has its own forme and must be traded around to get the forme you'd want.

In Pokemon Diamond, Pearl, and Platinum, there are meteorites in Veilstone City on the east-south of the city.

Visit Nacrene City's museum in Pokemon Black(2) and White(2) and have Deoxys be in the first slot of the party and let it touch the meteorite to change forme.

You can change its forme, also, in X and Y games. You will need to visit the museum in Ambrette Town and let Deoxys interact the meteorites.

In Pokemon Omega Ruby and Alpha Sapphire, you can go to Professor Cozmo's house in Fallarbor Town and interact with the meteorites.