How to use your Windows 7 / 8 or 8.1 Product key to activate Windows 10. Activation in Windows 10 - Microsoft improves activation process in Windows 10 during the free upgrade period which ends on July 29th, 2016.: "We have received a lot of feedback from Insiders on making it easier to activate Windows 10 on devices that take advantage of the free upgrade offer to genuine Windows by using existing Windows 7, Windows 8 or Windows 8.1 product keys. If you install this build of the Windows 10 Insider Preview on a PC and it doesn’t automatically activate, you can enter the product key from Windows 7, Windows 8 or Windows 8.1 used to activate the prior Windows version on the same device to activate Windows 10 by going to Settings > Update & security > Activation and selecting Change Product Key. If you do a clean install of Windows 10 by booting off the media, you can also enter the product key from prior Windows versions on qualifying devices during setup."
Additional information: A) Starting with the November update, Windows 10 (Version 1511) can be activated using most Windows 7, Windows 8, and Windows 8.1 product keys. For more info, see the section Activating Windows 10 (Version 1511 or higher) using a Windows 7, Windows 8, or Windows 8.1 product key in this topic.
B) 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.
C) If you previously installed and activated Windows 10 using a product key: You’ll need to enter the product key during reinstallation.