When your air conditioner isn't working well, it usually means that something is dirty or not properly situated. If heat can't escape from the evaporator coil, the air conditioner can't effectively cool the air. If the filter is dirty, the fan won't be able to blow cool air into your house. A little basic maintenance will usually take care of the problem.
Turn off the air conditioner at the main switch. There should be a switch near the unit that will turn off the power. If you can't find one, simply turn it off at the circuit breaker.
Clear out the space around the condenser coil. Remove any plants, furniture or other objects that are within 3 feet of it. Make sure there is nothing directly above it, since this can stop hot updrafts from channeling heat away from your condenser coil.
Use an old hair brush, broom or other stiff-bristled brush to clean dirt and leaves from the condenser coil. Built-up grime can insulate the coil, preventing heat from escaping.
Spray the air conditioner coils with an outdoor coil cleaner (see Resources). Follow the directions on the coil cleaner. Depending on the cleaner, you may have to mix it with water beforehand or spray it off afterwards.
Replace the filter. In window AC units, you can usually just open a flap and slide the old filter out and slide in a new one. In larger systems, you may need to unscrew a cover plate to get to the filter.