When you're logged into Amazon.com, click the "Account & Settings" tab (your name is on it), and scroll down and click "Your Content and Devices."
Now, on the "Manage your Content and Devices" page, click the SETTINGS tab on the right.
Click "Saved Wi-Fi Passwords" in the list.
Click "DELETE."
You should get a message that says "success!" and the delete button will grey out.