Your Mac must be one of the following models:
- iMac (mid-2007 or newer)
- MacBook (late 2008 aluminium, 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)
You can find out if your current Mac qualifies by clicking the Apple icon at the top left of your screen, choosing About This Mac, then clicking More Info.
Open the Mac App Store from your Dock to download OS X Yosemite. Then follow the onscreen instructions to install it. Return to the Mac App Store to download OS X Server.