Used incorrect email for cloud backup... need help changing it

I have upgraded storage that I purchase monthly under a master email address. My pro and my iPad and phones all back up to the cloud. I just purchased a new iPad and my son helped me set it up. We used a secondary email account to set the iPad up. Now I am out of data allowed for cloud backup for this new iPad and realized that it is not backing up with my master email account that I pay for space. It is using the “starter” allocation for the new iPad. I assume because of the secondary email address that I used. Now how can I change this on the iPad?