https://www.bleepingcomputer.com/news/microsoft/you-can-still-upgrade-to-windows-10-for-free-heres-how/

Back when Windows 10 was first released, Microsoft allowed users of Windows 7 or Windows 8.1 to upgrade to Windows 10 for free. Microsoft officially ended the free upgrade offer in December 2017, but a method still works that allows you to upgrade an older version of Windows to Windows 10 for free.

Microsoft doesn’t talk about that Windows 10 free upgrade offer anymore, but devices with legitimate Windows 7 or Windows 8.1 licenses could be upgraded to Windows 10 for free using the Windows 10 Media Creation Tool.