Diagnosis
The filter is behind the front grille, which doesn't intuitively pull off. It's a two-step process.
How to Fix It
Here's how to do it. 1) Unplug the unit. Always start there. 2) Look at the top of the front grille. You'll see two small, recessed tabs. Press inward on both of these tabs at the same time. 3) While holding those tabs in, pull the top of the grille gently toward you. It will hinge out from the bottom. 4) The gray foam filter is right behind it. Lift it out by the top edge. 5) Wash it in lukewarm water with a mild detergent, rinse thoroughly, and let it air dry completely before putting it back. Slot it back in, then snap the grille closed.
If That Didn't Work
If the grille won't budge, double-check you're pressing the correct tabs. They're small and near the top corners. Don't force it; you could break the plastic clips. A flashlight can help you see them better.
Heads Up
Never run the dehumidifier without the filter. And make sure it's bone-dry before reinstalling. A damp filter can grow mold and restrict airflow, causing performance issues.