Yosemite is only available for people who had downloaded it previously, otherwise it is no longer available.
You could probably upgrade to macOS Sierra if your Mac qualifies, otherwise you will be able to update to
OS X Yosemite (/ j oʊ ˈ s ɛ m ɪ t iː / yoh-SEM-it-ee) (version 10.10) is the eleventh major release of OS X (now named 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. Yosemite was released to consumers on October 16. Aug 13, 2015. The Mac App Store is built into OS X Yosemite, so it’s easy to get the apps you want. It’s just one click to download and install. Oct 16, 2014.
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.
iPhoto and Aperture if you had it are no longer supported by Apple in favour of their new Photos app.
Check compatibility
You can upgrade to OS Yosemite on any of the following Mac models. Your Mac also needs at least 2GB of memory and 8GB of available storage space.
MacBook introduced in 2009 or later, plus MacBook (13-inch, Aluminum, Late 2008)
MacBook Air introduced in late 2008 or later
MacBook Pro introduced in mid 2007 or later
Mac mini introduced in early 2009 or later
iMac introduced in mid 2007 or later
Mac Pro introduced in early 2008 or later
Xserve models introduced in early 2009
MacBook Air introduced in late 2008 or later
MacBook Pro introduced in mid 2007 or later
Mac mini introduced in early 2009 or later
iMac introduced in mid 2007 or later
Mac Pro introduced in early 2008 or later
Xserve models introduced in early 2009
To find your Mac model, memory, storage space, and macOS version, choose About This Mac from the Apple menu . If your Mac isn't compatible with OS X Yosemite, the installer will let you know.
Make a backup
Before installing any upgrade, it’s a good idea to back up your Mac. Time Machine makes it simple, and other backup methods are also available. Learn how to back up your Mac.
Get connected
It takes time to download and install OS X, so make sure that you have a reliable Internet connection. If you're using a Mac notebook computer, plug it into AC power.
Download OS X Yosemite
For the strongest security and latest features, find out whether you can upgrade to macOS Catalina, the latest version of macOS.
If you still need OS X Yosemite, use this link: Download OS X Yosemite. A file named InstallMacOSX.dmg will download to your Mac.
Install the macOS installer
Double-click the downloaded file to open a window showing its contents. Then double-click the file within, named InstallMacOSX.pkg.
Follow the onscreen instructions, which will guide you through the steps necessary to install.
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Begin installation
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After installation of the installer is complete, open the Applications folder on your Mac, then double-click the file named Install OS X Yosemite.
Click Continue and follow the onscreen instructions. You might find it easiest to begin installation in the evening so that it can complete overnight, if needed.
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Allow installation to complete
Apple Os X Yosemite Download Mac
Please allow installation to complete without putting your Mac to sleep or closing its lid. Your Mac might restart, show a progress bar, or show a blank screen several times as it installs both OS X and related updates to your Mac firmware.
Learn more
- OS X Yosemite won't install on top of a later version of macOS, but you can erase your disk first or install on another disk.
- You can use macOS Recovery to reinstall macOS.