Relax right where you are. Close your eyes and visualize yourself in a pleasant setting, perhaps standing on the beach watching a beautiful sunset or fishing in a mountain stream. Imagine the sights, sounds, smells, and touch of the place. Notice how relaxed you are there.
Take a break. Get some exercise or fresh air, or go somewhere private and yell or cry.
Ask yourself whether it's worth being upset over the situation. You can choose to stay calm and ignore it. If the issue is important, confront it directly, talk it out with a sympathetic friend, or write it out in a letter that you do not send.
List all the things you think you need to do right away. Then prioritize the list and do only the top few. The rest can become first priority tomorrow.
Do it now. We all know how thinking about the work we have to do and doing it makes us ever so much more miserable than just doing the work. So instead of putting it off until tomorrow, do yourself a favor and get to it.
Breathe. Deep breathing is extremely calming. Fill your lungs to capacity, imagine your neck and shoulders relaxing, and slowly exhale.
Neck rolls or shoulder rolls. Rolling your head in a circle several times and/or rolling your shoulders in a circular motion provide instant stress reduction.
Sleep. 7-8 hours a night if possible, or take a nap