Windows 10 was a free upgrade for any user who had purchased Windows 7/8/8.1 till July 29, 2016. Thereafter, it allowed accessibility upgrade to a few users. Again, that offer on December 31, 2017. Thus, as of present, Microsoft is only permitted paid upgrades to Windows 10 and the company hasn't officially mentioned any changes in near future, but we do not expecte Microsoft to offer it free again.

Are Windows 10 Updates free?
All Windows 10 updates and Windows 10 feature updates are free for anyone who has purchased the license genuinely or upgraded before the due date.

What if I do not like Windows 10 and wish to go back to an older operating system?
While we strongly advise not to downgrade from Windows 10, if you really wish to do so, go to Start >> Settings >> Update and Security >> Recovery. Click on Get started and select Go back to the previous version of Windows 10.