1. Snow Leopard (OS X 10.6, 10.6.7, 10.6.8 and up) - Check to see if your Mac is capable of running OS X Yosemite. If it is, perform a backup of your Mac. Go to the App Store and find OS X Yosemite, then double-click to begin installation.
2. Lion (OS X 10.7) – Check to see if your Mac is capable of running OS X Yosemite. If it is, perform a backup of your Mac, go to the App Store and find OS X Yosemite, then double-click to begin installation.
3. Mountain Lion (OS X 10.8) - If your Mac is running OS X Mountain Lion, it will be able to run OS X Yosemite without having to check capabilities. Perform a backup of your Mac. Go to the App Store and find OS X Yosemite. Double-click to begin
installation.
4. Mavericks (OS X 10.9) – If your Mac is running OS X Mavericks, no checking will be needed as it is capable of running OS X Yosemite. Perform a backup of your Mac. Go to the App Store and find OS X Yosemite. Double-click to begin installation.