You'll ideally need the pentax focusing screen tweezers; they'll release the catch and let you pull the screen out through the lens throat by a little tab.
Hold it up by the tweezers and use a rocket/hurricane blower, blow like mad, blow the inside of the prism/mirror box, lay screen in retaining clip, push clip until it locks. NEVER touch the focusing screen with a cloth, q-tip, brush or moisture, or of course, fingers...
it's a delicate operation that risks you scratching the screen, mirror or prism. be careful.
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