Diagnosis
You're right to clean it—a dirty filter is the number one cause of reduced airflow, inefficiency, and coil freeze-up. The filter is reusable and meant to be cleaned.
How to Fix It
1) Power off and unplug the unit. 2) The front grille should pull off—grip it at the bottom and gently pull it forward. 3) The rectangular foam filter slides out. 4) Take it to a sink and rinse it thoroughly under lukewarm water. Don't use soap or harsh chemicals, as the residue can get pulled into the system. 5) Gently squeeze out the excess water (don't wring it) and let it air dry completely before reinstalling. This can take 24 hours.
If That Didn't Work
If the filter is torn, heavily degraded, or still seems clogged after rinsing, you can order a replacement directly from Friedrich. The part number is usually on the filter frame itself.
Heads Up
Never run the dehumidifier without the filter. You'll let dust and hair coat the evaporator coil, which kills efficiency and can lead to much bigger problems.