The most common one would probably be
Chill out.
This is both informal and natural. Of course it's often likely to be poorly-received since it's not an instruction that's easy to follow.
It's common enough to have an entry in Merriam-Webster.
That definition provides an alternative:
Calm down.
That one is less informal but still completely natural. However it's not quite as apt if you want someone to lower their vigilance, rather than descend from a state of agitation or anxiety.
Note that the quotes in this answer can (edit: I incorrectly wrote should) end in a "!" rather than a "." but I find that doing that in conversational text (e.g. in a chat app) comes off as overly emphatic or imperious.
EDIT:
Another informal expression, which may sound patronising:
Cool your jets.