You have two ways to get Windows 10 activated for free, assuming you will do it within next four weeks. After July 29 it will always cost you:
Originally Posted by tns1
1.) In-place upgrade from a valid, genuine and activated Windows 7, 8 or 8.1
- Windows 7 Home editions and Windows 8 8 / 8.1 will be upgraded to Windows 10 Home, Pro requires a purchased product key
- Windows 7 Professional and Ultimate, Windows 8 / 8.1 Pro will be upgraded to Windows 10 Pro
2.) Clean install using a valid, genuine product key of Windows 7 or 8 / 8.1
- Windows 10 edition will be according to same principle as above in 1.
In addition you can always change / add a valid, genuine product key in a non-activated Windows 10 using Settings app.