The dust you can see in the viewfinder is typically dust on the focusing screen. It is located below the viewfinder prism, facing the mirror.
You can try cleaning it with a blower, or a soft brush. Do note that it is very fragile so do not touch it directly with any hard object.
Some dust may still remain on the other side of the focusing screen, if it bothers you then you will need to send it to nsc to clean.