How to extract HPKP (HTTP Public Key Pinning) backup pin?

I am doing HPKP when communicating on my asp.net server However, there is the inconvenience of changing the ssl pin key set in the server whenever the certificate is changed. When using hpkp, if you set the backup pin, it seems that there is no need to change the pinkey even if the certificate is changed. I don't understand how to extract backup pins. need help