calm down / calm (someone) down
Definition: to become less upset, angry, excited or lively. / to make someone less upset, angry, excited or lively.
E.g.1: He was very upset about it, but he calmed down eventually.
E.g.2: Calm down! You’re getting too excited!
E.g.3: She was very angry so he had to calm her down before they could discuss the problem properly.
This phrasal verb can be separated. (E.g. 3)