Many of us are considering ideal techniques to strengthen the lungs given covid-19. There is a mix of information, much is which is largely untested, but nonetheless might be useful.
Here’s something recommended by the NHS, it is likely to increase lung capacity. The recommendation is to inhale slowly and deeply. At the end of the inhale stop, hold for 2-3 seconds and take in a little sniff to further increase the air in the lungs and then exhale.