I have bought a used futon/couch from someone who had a big dog in his house. Is the foam and the frame/handle of the futon is najis? How to clean it?

The saliva and perspiration of a dog is Najis [impure]. In the above case, one does not know whether the dog has soiled the futon or not. As a matter of precaution, wash or wipe the Futon.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai