These pages use the default page template, so you don’t really need to do much to set them up. To begin, go to Pages > Add New to add a page. If you imported demo content, find the name of the page under Pages > All Pages and click on it.
To change the subhead that appears under the main title, while in the editing screen, locate Excerpt in the bottom right-hand menu (you might need to scroll down). Click on it to type in your new subhead.
To change the featured image, locate Featured Image in the bottom right-hand sidebar (you might need to scroll down). Click on it to open the Media Library and upload an image.
To add content to the body of the page, simply start typing or adding images to the page. To add a block, click the black plus sign.