Generate a correct and concise answer for the question based on the words in the context.

Context : 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. 
Question : The Windows 10 is based upon what?
Windows 8.1