You can change between design, preview and manage by clicking the relevant button at the top left of the Website Builder window. This article briefly explains the differences between the views. To access the views:
Scroll down to the Create your website section and click on the Website Builder icon.
Click on the Login button next to the relevant package.
The view buttons are now visible in the top bar of the window
The Mobile Site view will allow you to design and edit how your site will look on mobile browsers.
The Design view will allow you to add, amend and remove elements from your website. This is the default view when you first start up Website Builder and the view you will use to edit the visual look of your website.
The Preview view allows you to see your website as it would look when published on the internet.
The Themes view allows you to see what your website will look like with different themes.
The Manage view allows you to alter the back-end administration of your website such as meta-data and the domains mapped to your Website Builder site.