If you need to relax in the moment start by inhaling for five seconds holding your breath for five seconds and then exhaling for five seconds.
Repeat this practice for a whole minute since focusing on your breathing will often help you feel more calm. However if breathing isn't working try lying down in a quiet dark place and closing your eyes, then listen to your favorite music or put on some light background music.
Take this time to think about something other than what's stressing you out since getting some space from the problem is often the best way to relax.
If none of this is working try going for a walk outside without bringing any technology since your phone or computer can be stressing you out even if you don't realize it.