Click on your Profile Icon on the top-right hand site, then click on MY WORKS on the dropdown
Hover your mouse on PUBLISH. Then click on MY PUBLICATIONS
You will see all your published books
Locate the book you'd like to edit or update it's cover. Click on the PENCIL icon in front of the book
The book edit page will open up.
Hover your mouse on the Cover to change it
Once changed, click on Republish to republish it