Here are the steps for upgrading an ebook on Amazon
Click the “Hello <your name> to create a drop down menu
Click on “Your Content and Devices” about half-way down the list
Go the far left hand side of the screen that appears.
Beside the “Show” word is a drop down box. Select “Books”
A list of books you’ve purchased will appear.
Scroll down to find the ebook title you want to upgrade.
Click on the upgrade link below the title. (This used to be a small orange button but they’ve changed it) March 2020.
That’s it. The new version of the ebook will appear in your e-reader very quickly.