Here are some breathing exercises from the American Lung Association - https://www.lung.org/lung-health-diseases/wellness/breathing-exercises

Also, some other reference links on breathing, one for COVID but would still may be helpful. The last link mentions straw breathing which focuses on exhaling through a straw which I thought was interesting :
-- Coronavirus Recovery: Breathing Exercises: https://www.hopkinsmedicine.org/health/conditions-and-diseases/coronavirus/coronavirus-recovery-breathing-exercises
-- What exercises can help increase lung capacity?: https://www.medicalnewstoday.com/articles/323787
-- Breathing Techniques: https://www.takingcharge.csh.umn.edu/activities/other-relaxation-methods