Formal:
* Relax
* Unwind
* Rest
* Be at ease
* Compose yourself
Informal:
* Chill out
* Calm down
* Slow down
* Don't sweat it
* Don't worry about it
More specific:
* Relax: To enter a state of calmness and freedom from stress
* Unwind: To release tension and stress
* Rest: To stop working or moving to recover energy
* Calm down: To become less anxious or excited
* Slow down: To reduce your speed or activity level
* Don't sweat it: To not worry or get upset about something
* Don't worry about it: To not feel concerned about something
The best word to use will depend on the specific situation and the tone you want to convey.