To Import the demo content from .xml file, you need theme-demo-content.xml file which you will get from the downloaded folder (assets > import-data > theme-demo-content.xml) of WordPress Theme. If you want the theme to be installed as shown in demo, you need to import .xml file into your WordPress dashboard.
Go to Dashboard on the left Sidebar menu, Hover over the Tools and click on Import and scroll the page to the bottom and find WordPress. Click on Install now, after installing you will get Run importer, click on it and choose your theme-demo-content.xml file, and click on the Upload file and import button. Then assign your author from an existing user (or) a new user and click on Submit and your file is successfully imported. Now, you need to enable Menu, click on customize (present in the top bar), a left sidebar will appear. Go to Menus option, and then select Menu scroll to the bottom you will see Menu location select Primary, now your Menu is ready.