Upgrading from a Mac with OS X Snow Leopard to OS X 10.7 Lion is a snap. But before you start, make sure your Mac is prepared for the upgrade.
Your Mac will also need an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor.
Once it's ready, open the Mac App Store, find OS X Lion, purchase it, and start the download. The file is just over 4 GB and the amount of time it takes will vary. Count on at least an hour or more.
Once the download is complete, the OS X Lion installer app will be located in the Dock or Applications folder. Close any other open programs, and launch the Installer. Click Install.
Next, you must agree to Apple’s Software License Agreement twice. They want to know you really mean it!
Now verify the drive you want OS X 10.7 Lion to install upon. This should be the same drive to which you have OS X Snow Leopard installed. Typically, this is your local boot drive--Apple calls it the Macintosh HD. Click Install.
The system will ask for your credentials to verify the upgrade.
The system will prepare for the installation. The preparations only takes a couple of minutes. Then you'll have to reboot. When your Mac is back up, the installer will automatically start. It will take between 30-45 minutes to complete.
Once finished, your Mac will reboot itself into OS X Lion. You'll see the fancy new Finder desktop with the Andromeda Galaxy background.
If you're loading OS X Lion on a Mac with a built-in trackpad or an attached Magic Trackpad, a tutorial will then launch explaining how scrolling works. Don’t ignore it.
Scrolling defaults to the paradigm used in iOS, a startling change for Mac users. Up is down and down is now up. It takes some getting used to.