Microsoft has offered existing Windows 7 and 8.1 users free upgrade to Windows 10 version.
This change is for the Windows Insider Program ONLY, and is specific to how we’re delivering the Insider Preview builds. Available from July 29th onwards, users do not need an MSA to upgrade Windows 10 on their Windows 7 or Windows 8.1 systems if they are not receiving Insider Preview builds. Users will not be required to use an MSA on new PCs that come with Windows 10 preinstalled or clean installed from media. Some features in Windows 10 do require an MSA to use, such as downloading apps in the Windows Store.
More information is available here: