Diagnosis
The filter access isn't obvious on first glance. It's behind the front grille, which is meant to be removed by the user.
How to Fix It
1) Unplug the unit. 2) Look at the top corners of the front plastic grille (the part with the vents). You should see small indentations or tabs. 3) Gently but firmly pull the top of the grille forward until it unclips. It might feel like you're going to break it, but it's designed to snap out. 4) Once the top is free, lift the grille off. The rectangular foam filter will be right behind it. Rinse it under lukewarm water, let it dry completely, and snap everything back.
If That Didn't Work
If the grille truly won't budge, double-check you're pulling at the very top. Don't try to pry it from the sides. Some units have a small screw at the top center behind the control panel—peek up there with a flashlight.
Heads Up
Never run the dehumidifier without the filter. It'll clog the coils with dust incredibly fast, killing efficiency and potentially causing it to overheat or freeze up.