Microsoft launched the Windows Insider program a few months back. This program was for users to download and test the latest preview builds or as we call it the beta editions of Windows 10 which is expected to hit RTM status ie Release to manufacture status on 20th of July this year. Now a Beta version means, a developing and under testing version and may have serious bugs and must not be used on productivity machines. Many geeks and testers to get to know Windows 10 better have installed this and submitted various bug reports. User testing and bug testing is one the most important phases for software development and Microsoft needs to thank these guys who have submitted a lot of bugs, which would be used to improve the final product.
Many of these bug tester had demanded that Microsoft show some gratitude and provide them with a free copy of Windows 10. Finally Microsoft has heard them and has officially announced that all Windows insider program users will be eligible for a free copy of Windows 10 as soon as it launches. For this a Microsoft account has been made compulsory on the Windows 10 preview users. MSA or a Microsoft account has been made mandatory for receiving all future Windows updates. Here is the quote from Microsoft:-
Windows Insiders running the Windows 10 Insider Preview (Home and Pro editions) with their registered MSA (Microsoft Account) connected to their PC will receive the final release build of Windows 10 starting on July 29th. This will come as just another flight. I've gotten a lot of questions from Windows Insiders about how this will work if they clean installed from ISO. As long as you are running an Insider Preview build and connected with the MSA you used to register, you will receive the Windows 10 final release build and remain activated. Once you have successfully installed this build and activated, you will also be able to clean install on that PC from final media if you want to start over fresh.
So for now you can just head over and download the latest Windows insider preview ISO, install and connect your Microsoft account and wait for your free upgrade. Please note that the final Windows 10 will not force you to install and use a Microsoft Account. Once you activate Windows 10, you can delete the MSA. Also you can reinstall on that same PC as many times as needed, but you cannot move it to some other PC.