Do you know what fabric they're made of? Some are not water-safe (spot clean only).
When we got our couch, we specifically got one with cotton upholstery so we could wash it. Strip it, toss it (inside out) in a "casual" cycle (between normal and gentle), cold/cold with an extra spin cycle (but med speed, not super high), low soap, no softener. Dry it flat on a sweater dryer or over the porch rail.