Like all cameras with fixed pentaprisms and focusing screens, you have to perhaps get at the focusing screen via the opening for the lens and/or get at the focusing screen and pentaprism through removal of the top plate. Best left to a repair technician. Best thing to do is to send it in for a CLA (clean, lubricate, adjust) and ensure that old lubricants have not gummed up and prevent things from operating smoothly, and ask that the focusing screen and the pentaprism surface which is adjacent to the focusing screen are cleaned during the CLA (which they usually do as part of the CLA, but explicit request would ensure this is done!).
Originally Posted by Roundabout