Mindfulness
Here you can learn to breathe conciously, which is a proven tool to live a happier life.
4-7-8 Breathing
4-7-8 breathing is an easy but effective method to quickly feel calmer, I often use it when I feel on the verge of panic attacks, when I'm anxious in public or to relax. To start, inhale fully for 4 seconds. Then hold for 7 seconds (preferably, it's okay if you can't hold it for 7 seconds.) and then slowly release through your mouth as if you're blowing out a candle for 8 seconds. The GIF below can be downloaded and viewed locally if you prefer:
Box Breathing
Box breathing is another simple but effective technique. You simply breathe in for 4 seconds, hold for 4 seconds, exhale for 4 seconds and finally hold for 4 seconds before repeating. Again, the GIF below can be downloaded and viewed locally: