There are numerous breathing exercises you can find floating around on Facebook or YouTube. One technique borrowed from yoga is called a balanced breath. In a balanced breath, the length of the inhalation and the length of the exhalation is the same.
You can do this simply by counting; do not tense up or hold your breath:
Sit or lie in a comfortable position.
Inhale slowly to a number that is a comfortable length for you.
Exhale slowly to that same number.
Example: Inhale 1-2-3-4-5, exhale 5-4-3-2-1
Continue for several minutes or as long as you are comfortable. Do not force or strain the breath. If you lose your count, just start over. The number you choose is not important. The rhythmic breathing and especially the exhalation will bring release and relaxation.