How do you change rotom's form?

Question

I just want to know how

Answer 1

Pt/BD/SP: Use the Secret Key (Pt) or Rotom Catalogue (BDSP) in Rotom's Room, Team Galactic's Eterna Building

HG/SS: Rotom's Room in Silph Co.

B/W/B2/W2: Storage Room in Shopping Mall 9

X/Y: Professor Sycamore's Lab, 2nd floor, interact with boxes

OR/AS: Professor Birch's Lab

Su/Mo: Professor Kukui's Lab, interact with boxes

Sw/Sh: Use Rotom Catalogue obtained in Wyndon

PLA: Purchase Rotom's items from Ginter, interact with them in your quarters

S/V: Use Rotom Catalogue obtained from Porto Marinada auction

Rotom (Bulbapedia)

Answer 2

In D/P it´s impossible, in Pt using Secret Key (event item) and then going to Rotom room in Eterna Galactic building. In HG/SS is Rotom room in Silph co (you don´t need Secret Key but only having Rotom in front of party).

BW:
Route 9 - Department Store - storage