How do you make Landorus change forms in Pokemon?

You will need to have the Reveal Glass and confirm the changes to Tornadus, Thundurus, and Landorus. This item was first introduced in Pokemon Black 2 and White 2 in Abundant Shrine if you bring the Landorus in his Therian Forme, but you can find it in X, Y, Omega Ruby, and Alpha Sapphire as well. To find it in X and Y, you will need to talk to a scientist near the entrance to Shalour City in Reflection Cave. In Omega Ruby and Alpha Sapphire, you go to Mauville with Thundurus, Tornadus, or Landorus to a store owner, who will give you a Reveal Glass.