Yosemite is only available for people who had downloaded it previously, otherwise it is no longer available.

Apps compatible with Mac OS X 10.10 Yosemite Beta version of Mac OS X 10.10 Yosemite has been released to developers and the first million public testers. According to the official site of Apple, Yosemite will provide customers with a completely new user interface more pleasing to eyes, powerful built-in apps, a incredible connection with iOS. OS X 10.10, aka Yosemite, sports a more modern look and bridges the gap between Apple's desktop and mobile devices. The new Continuity helps you hand off tasks from iPhone to iPad to Mac, but that.

You could probably upgrade to macOS Sierra if your Mac qualifies, otherwise you will be able to update to

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Mac OS X El Capitan from here, Upgrade to OS X El Capitan - Apple Support

As both of these operating systems are a few upgrades newer than Mountain Lion you will need to prepare yourself.

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Make sure you have enough RAM, 4GBs is minimum. Make sure third party apps you use now are supported under El Capitan or Sierra and have updates available, https://roaringapps.com/apps?platform=osx. Also make sure that any peripherals such as printers, scanners etc are supported under El Capitan and Sierra check with the manufacturers of these items for updated drivers / software etc.

Apple Mac Os Yosemite Download

iPhoto and Aperture if you had it are no longer supported by Apple in favour of their new Photos app.