The content of all pages comes from the data folder (except for blog entries). After opening the data folder you will see all page contents file, like homepage.yml for homepage, about.yml for about page.
You can exchange the demo content with your own content, but remember that you have to follow the existing indentation in the demo content. If you miss the indentation will get an error. Copy the loop, and change your content. Every loop starts with - this symbol.