simmer down

Simmer down! (become calm after anger or excitement) — успокойся, не волнуйся; ~ не парься

Example 1: Just simmer down. I'm sure it's not as bad as you think it is.

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This idiom derives from simmer in the sense of "cook at low heat, below the boiling point."(Answers.com)

see also
[не парься!]