To publish a new version of a book in the Upload writing mode, you just need to upload a new book file, and that will automatically replace the old one.

To send "Release Notes" about the new version of your book to readers for a book published in the "Upload" writing mode, you need to go here:

...making sure to replace YOUR_BOOK with your book's unique web address.

Please note you shouldn't use the "Release Notes" option very often! This is meant only for major updates.