DIAGNOSIS You're right to be cautious. The exterior is straightforward, but the internal pre-filter is the main part that needs regular cleaning, and it's easy to mishandle.
FIX For the exterior, just use a dry or slightly damp cloth. For the important part: unplug the unit, open the front door, and remove the PECO-Filter. Behind it, you'll see a gray, mesh-like sheet—that's the washable pre-filter. Carefully peel it off its frame (it's held by adhesive strips). You can vacuum it lightly or rinse it under lukewarm water. Let it air dry COMPLETELY—this is crucial—before sticking it back onto the frame and reassembling everything. Do this every 3-6 months depending on dust.
IF THAT DIDN'T WORK If after cleaning and reassembly you get an error light or strange noise, you probably reassembled it out of order. The sequence is: ensure the pre-filter is dry and secured to its frame > insert the PECO-Filter > close the door > then plug in power. The unit does a self-check on power-up.
HEADS UP Never get the main PECO-Filter wet or attempt to clean it. Never use any detergents, soaps, or compressed air on any part, especially the sensor window. You'll ruin the coatings and sensors. Just vacuum, rinse (only the pre-filter), and dry.