Activating Windows 10 after the free upgrade from Windows 7 or Windows 8.1
If you upgraded to Windows 10 for free from an eligible device running a genuine copy of Windows 7 or Windows 8.1, Windows 10 will automatically be activated online and create a digital entitlement
for your device.
Activating after reinstalling Windows 10
How you activate Windows 10 after reinstalling it depends on how Windows 10 was originally installed on your device.
If you activated a free upgrade to Windows 10
or bought and activated Windows 10 from the Windows Store, you have a digital entitlement for your device
. This means you can reinstall the same edition of Windows 10 that your device has a digital entitlement for without entering a product key
. During reinstallation, if you’re asked to enter a product key, select Skip. Windows 10 will automatically activate online after the installation is complete.