If you have started building your website but now change your mind and want to change the template, you can easily do this. Just be aware that if you choose to change the template all the work you made to the site will disappear and you have to start over!
When you press the "Website" in the upper left once inside the Site Builder.
Then you get a menu, press the "Template Chooser"
Then you should see all the templates are available for your package.
You can also preview before you activate the template.
After you then press the "Use template" the template is active!