The focusing screen is located inside the camera's mirror box, the matte piece of plastic along the light path to the viewfinder. So it's not directly exposed, unless you start sticking things into your camera. Never try to touch it with your fingers (or even a cleaning cloth, for that matter) because it's an extremely delicate piece of equipment and is easily stained and scratched. If you want to clean it, use only a blower. If it doesn't work, leave it alone (it won't kill) or let the pros do it for you.
If it's scratched or stained, go the service centre and get it changed there. It shouldn't cost much more than $20 odd dollars.