You can change the theme to your website anytime by clicking on 'Dashboard' button at the top left corner of your screen.
After that simple select the desired theme and 'Use This Theme' button to select it, after that click on 'Change Theme' and the new theme settings and styling will apply to the current pages of your website.
Changing theme of your website will NOT erase any of the widgets/changes that you've made on your website. It will only apply the new themes settings. If you create a new page after changing to a new theme, then the page will have the new theme page templates.
If you wish to create a website having the same pages as the new theme, click on the 'Create New Site'. It will create a new 14 Day Trial website with the selected theme.
Once created a new website, you can always switch between website, edit and compare them. You can switch website by clicking the 3 dots on the left top.
After that a pop-up with your website will appear for you to switch between website or delete them.