I have changed my book cover and added to the book a few times. It will just show it as updated to people who have bought the book already. Here is my book: https://itunes.apple.com/us/book/cutest-book-ever!/id550927129?mt=11&ign-mpt=uo% 3D4
You’ll need to create a separate cover image and upload it to iTunes Producer. Use a tool like Photoshop or if you've created the book on your iPad you could just take a screenshot of your cover page and crop it.
The file must be a JPEG, PNG or TIFF file in RGB mode, (these are standard filetypes for images).
The image must also be a minimum of 1400px on its shortest side. It doesn’t matter if the cover art is landscape, portrait or square.