Unlike in Diamond and Pearl, Rotom has the ability to change form. However, this can only be done with the key item called Secret Key, only distributed by Nintendo events.
Once the key has been received from the man in the Poké Mart, go to Eterna City. Then, go to Team Galactic's base. Go to the top left corner of the first floor in between the wall and machine and press A. A new door will appear. Inside are some appliances. When you first walk in, you will notice five red appliances laying on the floor. The first is an oven, the second a fan, the third a fridge, the fourth a washing machine, and the last is a lawn mower.
Walk up to any of the objects and interact with it. Rotom will want to go inside the object. When it does, the appliance will disappear and Rotom will then be in the form of the appliance you choose. To change Rotom back to normal, go where the empty spot where the appliance should be and press A.
Special Notes: Rotom's type (Electric/Ghost) does not change with each form. The game will automatically change one move to be the special move.
Here are pictures for the original, fan, frost, heat, mow and wash in that order: