In other words, just because your Mac was compatible with El Capitan (OS X 10.11), Yosemite (10.10), Mavericks (10.9), or Mountain Lion (10.8) may not necessarily mean that you'll be able to upgrade to Sierra.
Make sure you’re logged into the correct i Tunes account for the specific update, and hit the dark gray Update button when you’re ready.Moreover, many third parties have stopped releasing software updates compatible with older operating systems.For example, the latest versions of Chrome and Firefox won't run on anything older than Mavericks.And wouldn't you know it - it comes with a bunch of new features. As well as a brand new name, which means after many years, "OS X" is no more. As has been customary since it stopped naming the OS after big cats, the latest operating system version has a moniker inspired by California and will be known as mac OS Sierra.Although the company started shipping OS X 10.11 El Capitan just last autumn, rumours about the next system have been around for some months. Apple has stopped referring to its Mac operating system as OS X, however.