This step is optional. It will add the demo data to your website that you might not expect (the existing content still remains, though). So, it’s best to import demo only on a fresh install.
Make sure you installed, activated the plugins and enabled the required modules before importing demos.
To import a demo, go to Appearance > Import Demo Data, select a demo and click Import button. It takes a couple of minutes to process all the content. After done, go to the frontend and enjoy the theme.