To activate Windows 10 Pro for free, you can use the product key from a previous business edition of Windows 7, 8 or 8.1. Also, if you were using a genuine copy of Windows 7 or higher, you can upgrade to Windows 10 without paying a penny. Normally, upgrading to Windows 10 pro requires some amount to spend on but still, if you try and follow some tricks, you can get it for free.
Helpful Link: http://getassist.net/how-to-activate-windows-10