Okay Diana, First you must make sure to kill the electric power to the unit. Most of the time there is an outside switch, but sometimes you have to throw the correct breaker in the panel. Next you should remove the end access panel. The most convenient way I have found to clean these is with a shop vac. Simply suck out all of the leaves and crud you find accumulated inside. Then when the muck is gone, you can take your garden hose and flush out the fins, from the inside to the outside. If you just have at it with a garden hose, and don't clean out the inside first, you will end up with a wet mash of muck on the bottom of your unit, which will foster rust and other undesirable things.