When you're re-playing events in your mind over and over, or worrying about things you can't control, acknowledge that your thoughts aren't productive. Thinking is only helpful when it leads to positive action.
If it's something you have no control over — like a natural disaster — think about the strategies you can use to cope with it. Focus on the things you can control, like your attitude and your effort.
Remember that your emotions will interfere with your ability to look at situations objectively. Take a step back and look at the evidence. What evidence do you have that your thought is true? What evidence do you have that your thought isn't true?
Incorporate 20 minutes of "thinking time" into your daily schedule. During that time period let yourself worry, ruminate, or mull over whatever you want.
When your time is up, move onto something else. And when you start overthinking things outside of your scheduled thinking time, simply remind yourself that you'll need to wait until your "thinking time" to address those issues in your mind.
Just like any other skill, mindfulness takes practice, but over time, it can decrease overthinking. There are classes, books, apps, courses, and videos available to help you learn mindfulness skills.
Change the channel in your brain by changing your activity. Exercise, engage in conversation on a completely different subject, or work on a project that distracts you. Doing something different will put an end to the barrage of negative thoughts.