I tried to look up some different breathing exercises for this post and there. are. so. many.
My personal favorite is just a nice deep breath, hold it for a moment, and let it out slowly. Long, slow exhales can stimulate the vagus nerve and send a calming message to your nervous system. In turn, this can lower your heart rate and blood pressure.
There are different breathing exercises out there for helping with anxiety attacks, stress, and calming… as always you can do a little research to find different techniques that suit you.