When editing the book you'll see this "crop" option above the coverage image:
Click that and you'll see some further options to modify the image including a rotate option. Make the change then hit save.
For new trials and paid subscriptions started after 19th August
You can crop and rotate manually uploaded images. Click on the "+" button below the coverage item to upload your own image
You can then crop, and rotate the image before saving and uploading the final edited image. Once your saved image has uploaded you won't be able to edit it again, but you can delete and re-upload it if needed.