Dust on the focusing screen usually does not affect image quality or exposure metering, so cleaning it is often optional unless it bothers you in the viewfinder. If you do clean it, start gently: turn the camera off first to reduce static, then use a hand blower to remove loose dust. A soft brush may help, but brushes can shed bristles and create more debris. Avoid wiping the focusing screen with a lens cloth, since it can make things worse or damage the delicate surface. If needed, the focusing screen can be removed and cleaned with a blower, but this can easily introduce more dust or cause damage if done carelessly. If you’re not confident, the safest option is to take the camera to a repair shop or service center.