It depends on why you are angry. If the boss has done something that has made you look bad, or done something that you can talk about--sit him/her down and be honest but respectful about what has happened. Always be honest but keep you tone and demeanor respectful.
This happened to me years ago. My boss was irritated at me for something (don't recall what) and got angry with me in front of a customer. Afterward, I took him aside and told him he was right, I had screwed but please in the future don't discuss it in front of a customer. He apologized to me because he knew I was right and I never showed him any disrespect.