hi there:
The free upgrade to Win. 10 from Win. 7 has ended years ago. But here is what I found.
As part of Windows 10’s November update, Microsoft changed the Windows 10 installer disc to also accept Windows 7 or 8.1 keys. This allowed users to perform a clean install Windows 10 and enter a valid Windows 7, 8, or 8.1 key during installation. Windows 10 would then report that key to Microsoft’s servers, and Windows 10’s activation servers would give your PC a “digital entitlement” (now a “digital license”) to continue using Windows 10 for free, just as if you had upgraded.
This also works from within Windows 10. Even if you don’t provide a key during the installation process, you can head to Settings > Update & Security > Activation and enter a Windows 7 or 8.1 key here instead of a Windows 10 key. Your PC will receive a digital entitlement.
hope that helps