The Windows 10 update tray icon on your desktop

If you are running a Windows 7 or 8.x machine, you might have noticed that starting in early June, your computer started showing a new system tray icon that would become the Windows 10 installer if clicked. In June, Windows 10 wasn't yet ready, so you simply enlisted for a free upgrade and download of the new version once it is available.

Now that Windows 10 is available, these opted-in computers have started to upgrade themselves. This is a great way to get Windows 10 for free. Latest information is that this free upgrade period lasts until July, 2016, which means you have roughly a year to upgrade.