If you are a member of the Windows Insider program and have the preview version of Windows 10 installed on your PC, you should be able to gain access to the final RTM (Release to Manufacturing) build of the OS when it is released sometime in 2015.



From Windows Central:

When asked, "will we be able to update to the RTM version when it ships from the TP branch?", Aul's answer was, "Yes, that is our intent." However, there's no definitive word yet if those same users will have to pay to update to the Windows 10 RTM build. There are some rumors that Microsoft will offer Windows 10 for free for users who have Windows 7 or higher installed but the company has yet to comment on pricing plans for the OS.

  Windows 10 testers should get an easy path to the release version in 2015