Some cameras have a removable vewfinder-eyepiece that allows for prescription dioptre lenses to be fitted. Try uncrewing yours and cleaning inside the bit you can reach.
Use a lens cloth and/or cotton ear-bud - wipe carefully, then blow off using a rocket blower or a bit of 'huff' if you're not a drooling idiot like me...
As others have said, be very careful about trying to clean the mirror and focussing screen (both accessed through the lens-mount on the front of the camera).
You may end up depositing more dust there or worse, scratching it...