Cool out
Definition 1 of 1
Pro Tip 1/3Informal Usage
Best used in casual conversations with friends, not in formal settings. 
Hey, let's just cool out and watch a movie tonight.
Pro Tip 2/3Emotional Shift
Used when someone needs to shift from a high-stress state to a more relaxed one. 
You seem tense, maybe you should cool out for a bit.
Pro Tip 3/3Subtle Reassurance
Implies support or understanding when someone is upset or angry. 
I get why you're upset, but let's cool out and talk it over.