Answer 1

There is a Meteorite in the Meuseum. Interact with it with Deoxys and Deoxys will cycle through its formes.

Comments

Where would you put Deoxys? At the front of your party or not?

Answer 2

Go to Nacarene city and go inside the Meuseum. Put Deoxys at the front of your party and go to the Meteorite and interact with it. Deoxeys will now cycle through its forms.