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
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.
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
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.
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.
Allow installation to complete
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.