There are cases that you want to re-install Windows from scratch due to some issues encountered or maybe you might want to install a fresh copy of Windows on your newly replaced SSD. You can download the Windows 10 ISO from Microsoft Website (https://www.microsoft.com/en-us/software-download/windows10) but in order for you to install it, you might need a Product Key. There is an easy way to retrieve a product key from your existing original Windows 10 installation. Just follow the instructions below.
Open the "Command Prompt" as "Administrator".
On the prompt, type the command below.
wmic path softwarelicensingservice get OA3xOriginalProductKey
You should now be able to retrieve the Windows 10 Product Key and re-install from a clean copy of Windows from Microsoft.
Note: This method displays the Windows product key stored in your computer’s BIOS or UEFI firmware. In other words, it shows the original Windows key your computer came with.
Updated as of October 2020.