Cleaning your air conditioning unit every few years is a good idea. It will make it more energy efficient and will make it last longer, avoiding costly breakdowns. This is, however, a powerful unit and safety needs to be a major consideration before anything else.
First of all, make sure there is no electricity flowing to or from the unit. There is normally a small power box near the unit, and that needs to be turned off. Also turn off the circuit breaker connected to the unit inside the house in the main power box. If you want to be absolutely sure, use a meter to make sure there is no electricity circulating.
Remove the fan cage carefully. Use screwdrivers and a wrench to loosen the fasteners, and lift it away carefully. Do not let your hands be in the way of the fan blades. it is unlikely, but there could be a bit of power left in the system, and it could turn when you lift it out.
From there it is a matter of cleaning out leaves and dirt. Use low to moderate water pressure to clean the unit. Clean and straighten the fins. Then it is important to clean around the unit to keep pests from building nests there. It is also a safety measure to keep the area around the unit clean.
Once you have cleaned the system and allowed it to dry, restore the electric power and test the unit to make sure it is working properly.
While you can save some money doing this yourself, it is also advisable to get your unit checked by a professional from time to time. If you have a leak, for instance, you will need a professional to take care of that problem. Clean ducts and air flow issues are also things a professional should be utilized. A key safety measure is to understand what you are capable of and to not go beyond those capabilities.