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. If you have iMac (Mid 2007 or newer), MacBook (Late 2008 Aluminum, or Early 2009 or newer), MacBook Pro (Mid/Late 2007 or newer), MacBook Air (Late 2008 or newer), Mac mini (Early 2009 or newer) or Mac Pro (Early 2008 or newer) - you can OS X Yosemite download and instal for free now via Mac App Store and Torrent.
Yosemite is only available for people who had downloaded it previously, otherwise it is no longer available.
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You could probably upgrade to macOS Sierra if your Mac qualifies, otherwise you will be able to update to
Mac OS X El Capitan from here, Upgrade to OS X El Capitan - Apple Support
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As both of these operating systems are a few upgrades newer than Mountain Lion you will need to prepare yourself.
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.
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iPhoto and Aperture if you had it are no longer supported by Apple in favour of their new Photos app.