Click on the cover image or click the "image" button above the book to open the cover image sidebar. Click "replace" to choose a new cover photo.
You can choose a cover photo from this book, from your photo gallery or you can upload an image from your computer (click my gallery, then upload). Select an image by clicking the circle in the corner of the image, then hit the "replace" button.
You can click and drag the cover photo to adjust the alignment. You can adjust the brightness, saturation, contrast and hue using the adjust button in the sidebar. You can also rotate, crop or flip the cover image using the crop/rotate button in the sidebar.