If you go to Admin > Required Fields you can see that Model type is something that is locked. When you go to Admin > Models you can see that some Models come from the Incident IQ Catalog, and so are locked, while others are Custom, and can be altered. So you cannot change the the Model of a device, but you can obviously change what Model a particular Asset is.

@PBauchan 650c0fd mvsd is right that you can only change the visibility of the Incident IQ Catalog models. But you are able to create a custom models.