Get Calibre for the Kindle.
Get this Kindle plugin zip file.
Unzip the file, Open Calibre>Preferences>Plugins>From File>Unziplocation/Calibre_Plugins/K4MobiDeDRM_v04.5_plugin.zip
Choose the zip file K4MobiDeDRM_v04.5_plugin.zip
Customize plugin>Enter the Amazon Serial No
The serial no is located from Kindle>Home>Settings>Device Info>Serial No “B00E-” (example)
Hit Ok, close and restart Calibre.
The DRM is removed while importing the book to Calibre. So if you’ve already added the book, delete it from Calibre and readd it.