Mindful breathing activates your para sympathetic nervous system to reset and rejuvenate your brain. If our breathing tends to be shallow, frequent with a focus on inhalation and short quick exhalations, then the body knows we are stressed. Only when we are relaxed, safe and at ease is our breath able to deepen and slow and allow long steady exhalations with pauses between breaths.
One simple method is 2 inhales to a count of four, hold your breath for a count of eight and then exhale slowly to a count of eight. Then notice the small still point, the pause, at the bottom of your breath before you inhale again.
Realize that you have the freedom to refocus and reset at anytime. As often as you desire, as a daily mindful practice and also in stressful times.