The best way to clean any garage door is to use a soft cloth and wash by hand. Do not use a pressure washer to clean the surface. Power washers and abrasive scrubbers will damage the surface of your door.
Use a mild soap like dish detergent and a sponge to wipe down your door. You can use a hose to wash off the suds. Be careful to not get water inside your garage. If your door is not sealed all the way, you can spray water directly in your home.