I can get it's original forme but I don't know how to change it.
>Starting in Platinum, Rotom also has five extra forms (each based on a household appliance), which can be accessed by possessing specific appliances.
Source: Bulbapedia/ Rotom
You can't change his forms in Diamond because he does not have any yet.
Rotom does not have any forms in diamond except the standard one, the other ones are from pokemon platinum you need pokemon platinum version in order to change the form