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. That's the word, at least, from Microsoft's Gabriel Aul when he answered a user's recent question on Twitter.
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.
Also, Aul's statement says that Microsoft's "intent" is to allow Windows Insider user to upgrade Windows 10 to the RTM build. That would also suggest he's leaving some wriggle room for the company to change its mind at some point on this plan.