A well-maintained older air conditioner beats high SEER-rated AC systems. If your AC comes with accessible air filters, you can detach them from the unit for cleaning. It is ill-advised to go in there to clean coils. You might cause more damage than you started with and hurt yourself. However, if you have the necessary know-how, remember to:
Use a gentle detergent.
Mix it with two parts of warm water.
Use a spray bottle to get the cleaning compound on the coils.
Give it enough time to sit, no less than ten minutes.
Use a soft cloth to wipe away any dirty buildups.