Diagnosis
The air intake filter is dirty, which will kill cooling performance and can cause the unit to freeze up or overheat. You need to access the rear grille.
How to Fix It
Unplug the unit. Look at the back side, where the exhaust hose connects. Above or around that exhaust port, you'll see a large plastic panel/grid—that's the intake grille. There are usually a couple of screws or plastic clips holding it. Remove them and gently pull the grille cover off. Behind it, you'll find a foam or mesh filter. Pull it out, wash it with lukewarm water and mild soap, let it air dry completely before putting it back. Reattach the grille.
If That Didn't Work
If the filter was missing or damaged, you might need a replacement. A missing filter lets dust coat the evaporator coil inside, which requires a more thorough cleaning with a vacuum and soft brush through the opened grille.
Heads Up
Never, ever run the unit without the filter. It's not just for air quality; it protects the internal coil from getting clogged with dust, which is a nightmare to clean and will permanently reduce efficiency.