As of May,2016, the Windows 10 operating system is running on 300 million devices (Link). Last one year Microsoft offered the free upgrade to Windows 10 from Windows 7, Windows 8 and Windows 8.1.
If you look at shopping websites, you could find so many laptops and desktop PCs are still available with the old Windows 8 or Windows 8.1 operating system. You could buy one of those device and upgrade it to Windows 10 at your own convenience. Now this is going to end.
Microsoft clearly said that there is no more extension of free upgrade period.
The free upgrade ends on 29th July 2016. After that, Windows 10 is available for separate purchase or comes pre-installed on a device.