What do you do if your cat is a demon?

first of all, no cat is a demon, there might be a demon in it but it is not a demon, so I'm not sure how to help you

A Little More Information
There is little likelihood your cat is possessed of a demon. There have been NO documented cases of said.
Cats have for centuries been associated with witches, as familiars and mistakenly accredited with being messengers or in league with "the devil". In some cases cats were brought to trial as co-conspirators in witch trials.
Cats are NOT demons, or possessed, they do have habits that most people do not fully understand. They are by nature nocturnal and predators and for all we like to think they are as civilized as we are, they are not.
Their senses (vision, hearing, smell, etc) are different from ours and so they tend to react to things we are unaware of. This often explains behaviour some people find unsettling.
It should be noted, that like people, cats can become ill, or inherit traits or weaknesses, that may effect the "personalities", if your cat has suddenly become aggressive, or is behaving erratically, the first this you should be doing is calling your veterinarian, not and exorcist.