Deep breathing (also known as diaphragmatic breathing, abdominal breathing, belly breathing, or costal breathing) is very beneficial to everyone. It fills up the lungs with air and therefore properly oxygenates the body, even at the cellular level.
Experts suggest normal breathing on average only fills up one third of the lungs with air, and therefore the entire body functions at a third of its capacity. Everyone should deep breath as often as possible every day. 
Here’s how:
Lie comfortably on your back. Put one hand on your chest and one on your stomach.
Slowly inhale through the nose and feel your stomach expand with your hand. Slowly exhale through the mouth and feel stomach contract.
Count to four, (four seconds) on each phase:
Inhale to a four-count, hold air in lungs for a four-count and exhale slowly to a four-count.
The key is to relax, slowly breath and feel the lung-action. If your shoulders rise when you inhale, you are not performing it properly. You are to flex the diaphragm rather than breathing shallowly by flexing your rib cage. This deep breathing is marked by expansion of the stomach (abdomen) rather than the chest when breathing. It is generally considered a healthier and fuller way to ingest oxygen.
Performing deep breathing regularly is therapeutic, and can become a standard way of breathing.