You need to be running Snow Leopard to order, download, and install Lion, Mountain Lion, and Mavericks, which are available through the Mac App Store.Although OS X 10.7 Lion, 10.8 Mountain Lion, and 10.9 Mavericks have each replaced it in turn, there are good reasons you may want to (or even need to) have Mac OS X 10.6 Snow Leopard.
