Visualization - imagine a very peaceful scene, like laying on the beach, out in a fishing boat on a lake, in a mountain cabin or whatever. It can be a real place or you can make it up. Try to invoke all your senses as you imagine being in this very peaceful, relaxing place. For example, you might imagine the sun on your skin, the cool breeze on your forehead, the salt tang of the ocean, the grit of the sand. Try all these methods and see which works better for you.
Some people do better with muscular methods, others with visualization.
All these can be learned quite readily and often work very well. In the long run, however, it is better to learn how to avoid getting stressed out in the first place. Well, the clue is in the visualization method. Thinking peaceful thoughts makes you feel relaxed. In imagining a peaceful place, you have also distracted yourself from whatever thoughts you were having before.