I believe you'll find that there are two pins that jut out holding the screen in place. Simply push them in with your finger and pop out the focusing screen. Sometimes you can just turn the camera upside down and it will fall out. If it is cruddy, a small screwdriver or fingernail file will get it out.
Clean it with windex or lens cleaning solution and place it back in. When you slide the viewer back in, it will automatically cause the pins to slide back in place.
When the screen is out it easy to clean the mirror at the same time.
Oh I should mention, it is not a good idea to spray windex or anything else into the camera. You're better off to spray it on cleaning tissue or a cloth and clean these parts that way.
Hope this helps,