If you want to change a website template, you will simply need to select a new one and connect it to your subscription. To do so, please follow the steps below.
1. Select a new website template as described here.
2. Connect that template to your plan as described here.
If the plan in question is already connected to any template, you can disconnect it as described here.
Note: The content will not be automatically transferred from one website template to another, though you can use the Page importer tool.