Are you not using Windows 10 yet? You only have until July 29, 2016, to upgrade for free.
After that, Microsoft is going to start charging for upgrades to the most recent operating system. If you're going to upgrade to Windows 10 Home, it'll cost you $119.
The company announced the end of the free upgrades in its blog post detailing the 300 million-device milestone for Windows 10.
(Only another 700 million devices to go over the next 24 months to make Windows EVP's Terry Myerson's stated dream during the Build 2015 keynote: "Our goal is that within two to three years of Windows 10’s release there will be 1 billion devices running Windows 10.")
If you're not convinced as to the necessity of upgrading to what's been a mostly-stable operating system, check out the Microsoft announcement, which features a video countdown of ten reasons to upgrade, as delivered by noted keynote showman, marketing manager Bryan Roper.