Relaxation has been long-known as a treatment approach for anger problems. Muscle relaxation, meditation, deep-breathing, etc. are part of almost any standardized treatment approach. One particular type of relaxation (though, it’s much more than that) is yoga, which includes all the important treatment components of relaxation.
For some specifics, take a look at this 2013 Huffpost article: Yoga For Anger: 3 Moves to Help You Calm Down