Diagnosis
The TH685P has a fixed, internal air filter that you cannot remove for cleaning. Dust is managed by the airflow system.
How to Fix It
Use a can of compressed air. With the projector off and completely cool, hold it upright and gently blow air into the intake vent (on the side) and out through the exhaust vent. Do this in short bursts to dislodge dust without forcing it deeper.
If That Didn't Work
For heavy dust accumulation, you may need to open the case, which voids the warranty. If overheating persists due to dust, professional service is the safe option.
Heads Up
Never use a vacuum cleaner directly on the vents. The static suction can damage internal components. Compressed air is the only recommended method.