If you previously performed the upgrade from Windows 7 to Windows 10 that was offered free for a limited time from Microsoft, and have not since changed motherboards, your hardware configuration should be listed on a Microsoft server as having a valid Windows 10 license, and upon reinstallation of Windows 10, either through an in-place upgrade or a clean install, should activate. Rolling your system back to Windows 7 did not invalidate your free Windows 10 license.
The recommended approach is a clean installation of Windows 10, using source media from one of the above links. In-place upgrades have historically been plagued by numerous issues and are generally recommended against when possible.
An initial in-place upgrade from Windows 7 to Windows 10 was necessary to register your computer's hardware configuration with Microsoft's registration system. Since that's been completed, you now have both in-place and stand alone installation methods available to you.