My iPhotos will not open in Photos on my Mac. How can I change this?

I assume it is because you just upgraded to Catalina. Photos replaced iPhoto 5 years ago and iPhoto will no longer run in Catalina.

Launch Photos with the option key held down. From the resulting dialog, choose your iPhoto Library. Photos will create a new library with your iPhoto Library media. Your old library will not be changed, though it will be renamed.