You might realize your mistake when you see the "Get for Kindle" button on your download page...
Go ahead and click the "Get for Kindle" button. This will open a new page or tab in your browser and lead you to Amazon.com's website. From there, look for a "Return Book" button.
Go back to the tab/window that contains the NHDB website >> Your Account >> Currently Checked out Items. The book should not be listed. If it is, wait a moment and refresh the page.
Learn to return a Kindle Book early back to the collection.