Synonyms for Calm down:

n.

mollify, appease, conciliate, smooth someone's ruffled feathers, propitiate, disarm, placate, pacify. control oneself, keep oneself under control, get organized, catch up, rest, look up, take it easy, get hold of oneself, jump, go easy, restrain oneself, ease off, change for the better/worse, calm oneself, get a grip on oneself, relax, come a long way, keep cool, recover, pick up, sharpen, improve. calm down (noun)
calm, cool it, cool off, still, lull, quiet, quieten, settle down, simmer down, tranquilize, tranquillize, chill out, tranquillise.

v.

compose oneself (verb)
cool it, simmer down, keep cool, restrain oneself, relax, get hold of oneself, rest, control oneself, go easy, take it easy, chill out, cool off.
emotion (verb)
simmer down, cool off, calm, settle down, chill out, cool it.

Other synonyms:

placate, pacify, mollify. disarm, appease. cool
simmer down.
kick back
take it easy, cool off.
quiet
quieten, cool it.
relax
ease off.
Other relevant words:
propitiate, take it easy, restrain oneself, get hold of oneself, go easy, appease, relax, placate, keep cool, ease off, pacify, mollify, calm oneself, rest, control oneself.