Intro: How to Upgrade to Yosemite From Mac OS X, Mountain Lion, Snow Leopard, or Mavericks
Before you install Yosemite be sure to check you have the right Mac. The following qualify: 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, Mac Pro, Early 2008 or newer, Xserve, Early 2009. Make sure your Mac also has: OS X Mavericks, Mountain Lion, Lion, or Snow Leopard v10.6.8, already installed2 GB or more of memory8 GB or more of available disk space.
Step 1: Download OS X Yosemite in the Apple App Store
Yosemite is available from the Apple App Store. Select App Store from the Apple Menu to open the App Store, then click the Download button for OS X Yosemite in the App Store window to begin installation.
Step 2: Enter Your Password and Finish Downloading
Enter your Apple ID and password to continue.
Once the installer has finished downloading, it automatically opens. Follow the onscreen prompts to finish installation.
Step 3: Let Your Mac Startup
Your computer may restart many times during installation to install related firmware updates. Once installation has finished, your Mac starts up from OS X Yosemite.