This easy-to-use app provides 4 breathing exercises as well as the functionality to create a custom breathing exercise.

478 breathing aims to promote relaxation as well as reduce stress and anxiety.
Box breathing helps to calm the mind and bring focus.
Coherent breathing aims to synchronize the breath with the body's natural rhythms. This in turn balances the autonomic nervous system, increases heart rate variability, and promotes relaxation.
Tummo breathing, the Tibetan Buddhist practice also known as "inner fire" or "psychic heat" meditation. It helps stress reduction, mental clarity and overall well-being.