Windows 10 Digital License When you upgraded from a previous version of Windows or receive a new computer preinstalled with Windows 10, what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers. The Windows 7 or Windows 8 genuine license you were previously running will be exchanged for a diagnostics key.
Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate. You will prompted to enter a product key a couple times through the installation, click I don't have a key and Do this later. There is no need to know or get a new key, but if you have issues with Digital Licensing or the Activation Troubleshooter, you can utilize an existing Windows 7 or Windows 8/8.1 product key you already own to reactivate if necessary. Or use the reset function in Windows 10. Reinstall by Performing a Direct Clean Install If Windows 10 was already installed and activated on your system, just proceed to perform a direct clean install as described in the following articles.