Clean the air filter.
On most models, the air filter is easily accessible by removing the front panel. Take the filter out and clean it gently with a combination of warm water and dish soap or white vinegar. Let the filter air dry completely before reinstalling it. These should be cleaned once per month during the cooling season. If you have pets, or allergies consider cleaning it more frequently.
If the filter is damaged, it should be replaced. Air Conditioner Filter Parts