Diagnosis
It's easier than it looks. The filter is behind the front grille, which doesn't have obvious latches. People often pull on the wrong part.
How to Fix It
1) Make sure the unit is off and unplugged. 2) Look at the very top of the front panel, right under the control area. You'll see two small indentations or slots, one on each side. 3) Press gently inward on the top of the panel right above these slots, then pull the *top* of the grille outward. It should hinge out from the bottom. 4) The large, rectangular foam filter is right there. Slide it out. 5) Wash it with lukewarm water and mild soap, let it air dry completely before putting it back.
If That Didn't Work
If the grille really won't budge, check for any hidden screws along the sides. Some models have one or two screws at the bottom holding it in place.
Heads Up
Never run the unit without the filter. It'll clog the evaporator coils with dust in no time, and that's a much harder fix. And make sure it's bone dry—a damp filter can cause mold and ice buildup.