Changing the theme without making a new site and paying for it

My Editor X site opened with a pre exisiting theme template.
I started a new one with blank canvas and when I go to add my custom
domain name to it the page goes to payment options again.
I already paid for premium. How do you change a theme once your in it
without making a new site and paying for it.

I change themes by create a new site, building it out and the migrating the subscription and domain name to it. Subscription first, then domain name assignment. I hope this helps.