DIAGNOSIS The front grille is designed to pop off, but it feels stiffer than it should. That's just poor design—the clips are tight. The filter is behind it and needs cleaning every few weeks for performance.
FIX Turn the unit off. Look at the sides of the front grille. There are no screws. Firmly grip the left and right edges of the grille and pull straight forward. It will unclip with a bit of force—start from one side. Once it's off, you'll see a large rectangular foam filter. Slide it out, rinse it under lukewarm water (no soap, don't scrub), let it air dry COMPLETELY, and slide it back in. Snap the grille back on.
IF THAT DIDN'T WORK If the grille truly won't budge, check the very top center. Sometimes a small, hidden tab needs to be pressed down first. You can also try inserting a flat plastic tool (like a guitar pick) into the seam on the side to gently pry the clip loose.
HEADS UP Never run the unit without the filter, it'll get clogged with dust internally fast. And make sure the filter is 100% dry before reinstalling. A damp filter can restrict airflow and cause icing on the coils.