On Route 9, there's a mall called Shopping Mall Nine. On the first floor, there is a door near the back with boxes and a janitor. Interact with the boxes with Rotom in your party to change its form. If you choose the Freezer, it will be Rotom-F. If you choose the Lawn Mower, it will be Rotom-C. If you choose the Washing Machine, it will be Rotom-W. If you choose the Oven, it will be Rotom-H. If you choose the Fan, it will be Rotom-S.