Windows 10 will be available as a free upgrade for millions of current Windows 7 SP1 or Windows 8.1 Update users, for a year (specs). This offer is extended to people with Genuine and non-Genuine Windows copies (Chinese pirates and others). As details a-plenty flood the internet, there is also the news that this will be the last Windows “version” as Microsoft shifts gears for its OS and moves to a subscription model.
WinBeta has more information on how to reserve your free copy of Windows 10, or get rid of that notification (cut to the chase: uninstall update KB3035583).
If you’re already running Windows 8.1 and want to test run Windows 10 now, you can try this tutorial to run a Windows 10 Technical Preview as a virtual machine (VM), or another tutorial that goes through VirtualBox, dual booting, and a stand-alone install. You can also try a Technical Preview on your Mac, if you want to peak at the new browser and digital assistant.