Change the filter. The filter on an AC unit is supposed to be changed once a month anyway. On a central-air system, you typically have to remove a metal panel to get to it. A window unit sometimes has a permanent filter that should be cleaned with a mixture of vinegar and water.
Clean everything else. Check that the condensation drain isn’t blocked, and use an algaecide if needed. Turn off power to an outside unit and clean the coils with a hose or a commercial coil cleaner if the unit’s particularly dirty. Wipe the fins off with a soft brush and remove any plants or debris blocking air circulation. If you have a foil-wrapped evaporator located above your furnace, clean that too.
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