all the content comes from content folder. The content folder is a subfolder of the project folder. After open the content folder you will see all page contents file, like about.md for about page, contact.yml for contact page.
You can easily change the demo content with your won content, just remember that you have to follow the indent that are provided in demo content. If you miss the indentation will get an error. So carefully copy the loop, and change your content. Every loop starts with - this symbol.