Head to the Silph Co., with Rotom at the front of your party (out of it's Pokeball). Enter the Silph Co. and you will find the place is deserted.
Go into the elevator, and you'll arrive at a mysterious room with appliance that match Rotom's alternate forms. Click on one of the appliance for Rotom's form to change. Then, voila! Your new formed Rotom. You can come back anytime to change it's form.
Hope I helped. :)