Dust visible in an optical viewfinder is often not deep “inside” the camera, but on the focusing screen or the bottom face of the prism/pentamirror area. On many DSLRs you may be able to see this area by removing the lens while leaving the mirror down.

For loose dust, start with the least invasive method: gently blot or wipe only very carefully with a soft cotton swab. Some users also clean that accessible surface with the same swabs and sensor-cleaning fluid used for sensors, but this can be awkward and should be done very cautiously to avoid damaging delicate internal parts.

So the practical advice is:

• confirm the dust is in the optical path, not on the outside eyepiece
• remove the lens and inspect the area above the mirror
• try the gentlest cleaning first
• if using swabs/cleaning fluid, use extreme care

If you are unsure or the dust is not easily reachable, professional cleaning is the safer option.