Yosemite is only available for people who had downloaded it previously, otherwise it is no longer available.
Download Mac OS X Yosemite - The OS X Yosemite 10.10.5 update improves the stability, compatibility, and security of your Mac, and is recommended for all users. OS X Yosemite (/ j oʊ ˈ s ɛ m ɪ t i / yoh-SEM-it-ee; version 10.10) is the eleventh major release of macOS, Apple Inc.' S desktop and server operating system for Macintosh computers. OS X Yosemite was announced and released to developers on June 2, 2014, at WWDC 2014 and released to public beta testers on July 24, 2014.
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
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.
iPhoto and Aperture if you had it are no longer supported by Apple in favour of their new Photos app.
Jul 15, 2017 5:07 AM