Keep your cool
Definition 1 of 1
Pro Tip 1/3Stressful Situations
Use 'keep your cool' when talking about handling stress or pressure without getting upset. 
Even when the project deadline was moved up, she managed to keep her cool.
Pro Tip 2/3Emotional Control
'Keep your cool' implies controlling your emotions so you don't show anger or frustration. 
Despite the rude customer, the waiter kept his cool and remained polite.
Pro Tip 3/3Informal Usage
The phrase is informal, suitable for casual conversations, not formal writing or speeches. 
In an argument, it's better to keep your cool than to shout.