Locate the featured image module on the edit screen and click set featured image. A pop over dialogue will appear where you can upload an image from your computer, or another website or URL. If you have already uploaded an image, you can choose it from the gallery or from the media library. Once you have uploaded it, you can click show to display various image options. Scroll down to the bottom to the list of options to select one to use as the featured image. You can click anywhere on the editor page in the background to close the pop over dialogue. You will see a thumbnail version of the image you chose as your featured image. If your website is set up with a theme or plugins that use this feature, this image will represent that post or page. Click on the preview changes button before publishing or click the publish button to publish the changes.