I'm not sure if this is advisable in
@Runum's situation but generally speaking longer exhales and shorter inhales slow down your heart rate. A good rate is 2 to 1 (an exhale that's twice as long as the inhale) This, plus focusing on the diaphragm. Most people breathe from their chest and that's not a full breath.
Box breathing is a very good exercise to master that. Depending on your abilities you start with, for example, 4 second inhale, 4 second hold, 8 second exhale, and 4 second hold and then repeat the cycle for a few minutes while focusing on your diaphragm.
But like I said above, while I assume it may be a great exercise for post-covid recovery when done slowly and mindfully, I'm not sure it's advisable now.