Hi there thank you for your question.
to improve your breathing it is important to
1) in most cases; practice to breathe less (less often and light)
2) breathe through your nose
3) learn how to use your diaphragm in an optimal way

I my course I cover this, and with it you can take the time to do the exercises with help of video and audio.
Of course it is also wise to find some personal help from a breath therapist.
Online you can look up Patrick Mc Keown who has more info on astma and breathing.

good luck!

I have astma and want to improve my lungs capacity. What video or sitas you advice me to watch / read? It’s hard to find information.

Thank you very much for sharing…