The first step is to recognize the mistake. Not to justify the anger even if there was a clear provocation. One must know it as a weakness that has to be conquered whatever be the challenge one meets on the way.
Do not try to change others. It is the most fruitless exercise. Learn to accept people as they are. Be generous knowing that none is perfect. This gives rise to love and compassion.
Do regular workouts and practice deep breathing and meditation to call in Peace and Equanimity.
Keep the will not to repeat it. This may or may not be possible to achieve in a single stroke. But the will must not give up and not accept the suggestion that we cannot do anything, are helpless etc.
Offer the anger to the Divine with a sincere will to get rid of it.
Read books that are inspiring and show the noble side of life, and avoid company of those who are full of anger, because whatever is cleansed by the Grace can return by contamination.