DIAGNOSIS You're right to clean it—a clogged pre-filter makes the main filter work harder and reduces airflow. But the machine is a risky move that can shrink or damage the fabric.
FIX The official, safe method is a gentle hand wash. 1) Unplug the unit and take the filter assembly out. 2) Carefully peel the fabric pre-filter off the main carbon filter—it's attached with Velcro. 3) Vacuum it first to get loose dust off. 4) Wash it by hand in lukewarm water with a mild detergent. Don't wring or twist it. 5) Rinse thoroughly until the water runs clear. 6) Let it air dry completely before you Velcro it back on. A dry pre-filter is crucial to prevent mold.
IF THAT DIDN'T WORK If the fabric is stained beyond cleaning or has gotten a tear, you don't need a whole new filter set. Blueair sells replacement pre-filter covers separately in different colors. Just swap the old fabric shell for a new one.
HEADS UP Never, ever put the main particle filter (the big white cylinder) in water. That will ruin it instantly. Only the outer fabric shell is washable.