The best way is to make a new site. Create the new site and begin building it with Websitebuilder. Your old site will be unaffected and still available for editing. When the new site is done, then move the domain to the new site. You can on the old site make new pages and then rename them all when done so the take the place of the old, but you can't update the template without eliminating the content, so the new site is the best method. Also, you will get the newest templates and editor. It will make a more modern site for you.