If its on bottom side you can try blowing it off, but don't wipe it as you will just mess up the focusing screen or the mirror. Regular canned air will work for something this simple if you don't have a rocket blower. If the dirt is on the top of the focusing screen its too big a job unless you are REALLY handy with small mechanical things and want to pick up a set of JIS screwdriver set and some odd spanners too as I recall. Get a light seal and bumper foam kit to put in if you are that handy.
You can gauge the condition of your seals and bumpers just by taking off the lens and opening the rear door where you load film, the door will have a strip of foam along the top and bottom on the inside, and with the lens off you will see a black foam strip on the upper inside of the light box (where the mirror is) that the mirror smacks into when it swings up. Both should be springy black foam, if either are sticky or crumbly the camera needs to go in for service.
Better in that case to send it to Eric for a full CLA service, I wanna say its less than $75 on that camera and a K1000 will give you another 20 years after that sort of service, they are easily repairable cameras, not meant to be thrown away.
Eric website:
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