It easier to prevent the stress than to calm down after it happens. Identify the triggers that upset you and make a strategy to deal with them to the prevent the stress. If stress come´s anyway, what works for me is to let out my emotions whatever they may be and to express myself either by talking outloud or writting. Letting out is the key. I'm sure there are other effective ways to deal with stress such as; meditating, sleeping/resting, working out, eating healthy. All of those are great too. Take care!!!