Windows 10 is a personal computer operating system released by Microsoft as part of the Windows NT family of operating systems. Windows 10 is the latest Microsoft Operating System released to consumers since July 29, 2015. To encourage the adoption of Windows 10, Microsoft announced that during its first year of availability, upgrades to the operating system would be made available free of charge to users of genuine copies of eligible editions of Windows 7, and Windows 8 after update to Windows 8.1.
However, not every Windows Operating System is up-gradable to Windows 10. On Microsoft Website it says:
Find out if your Windows is eligible for free upgrade to Windows 10."This free upgrade to Windows 10 is available for PCs and tablets. Qualified Windows 7 or Windows 8.1 devices can upgrade for free."