Assuming you mean cleaning bird poop off of an AC unit that is already installed:
First, put on some gloves to protect your hands. Next, use a hose to spray down the entire unit, paying special attention to the areas where birds tend to perch. If the poop is fresh and wet, it should come right off. If it's older and more dried on, you may need to scrub a little bit with a brush or cloth. Once you've removed as much of the poop as possible, rinse the unit again with clean water.