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 license for your device. This means you can reinstall the same edition of Windows 10 that your device has a digital license for without entering a product key. During reinstallation, if you’re asked to enter a product key, you can skip this step. Windows 10 will automatically activate online after the installation is complete.
Activating Windows 10 after a hardware configuration change
If you made a significant hardware change to your device (such as replacing the motherboard) Windows 10 might no longer be activated. If you're running Windows 10 (Version 1607) and added your Microsoft account and linked it to the digital license on your device, you can use the Activation troubleshooter to reactivate Windows. For more info, see Using the Activation troubleshooter