To upgrade from Mac OS X Snow Leopard 10.6, download and install Mac OS X 10.6.8 Update Combo v1.1 followed by Mac OS X Lion 10.7.5.
If you are unable to upgrade, see How to create a bootable installer for Mac OS X Lion below.
To upgrade from Mac OS X Tiger 10.4 or Mac OS X Leopard 10.5, first upgrade to Mac OS X Snow Leopard 10.6, then upgrade to Mac OS X Lion 10.7. * If your Mac shipped with OS X Mountain Lion 10.8 or later, it cannot be downgraded to Mac OS X Lion 10.7.