go to the Amazon website
select Content and Devices from the upper right menu there
log in
find the book
click on the More button to the right of the title
Select Download & transfer via usb and select the target device (this will download the files to your computer)
then
Locate the file either on your desktop or within the file browser – it’s usually in the Downloads folder
Turn on your Kindle and put in a pincode if you have one
Connect the Kindle to your computer via USB cable.
Open File Explorer
Locate the “Documents” folder within the Kindle.
Copy the file from the original location to the Kindle “Documents” folder.