By the way, on the newer models, the large chrome ring, in the picture, is just behind the plastic in the front, if that makes sense... is threaded and unscrews counterclockwise, (as normal), to remove the cartridge. Lube the new one with a touch of waterproof, or "plumber's" grease when reassembling. (In a pinch, a little spit will do. You don' want the O rings to bind up inside when sliding back in).
Jon