Set a ladder against the side of your home and have someone hold it while you climb. Use caution when walking and working on your roof. If your roof is too steep, get a professional to clean the a/c unit for you to avoid personal injury.
Vacuum the rooftop a/c unit and the area around the unit to remove any dust and dirt.
Spray the rooftop a/c unit down with a hose and a high powered nozzle to rinse off any stuck on dirt or dust the vacuum didn't get. (Ref 1 and 2)
Brush the fin comb through the coil fins on the outside of the a/c unit to return them to their original state.