Make sure all your application are closed and launch the Mac App Store from your dock.
Click to select Mac OS X Lion from the Top Paid list on the right.
Click the $29.99 button under the icon at the top left of the window.
Click the green Buy App button that appears.
You'll now be asked to login with your Apple ID and password. If you don't have click the Create Apple ID button. Enter your ID and password then click the Sign In button.
You'll notice Lion jump into your dock and download of the 3.74GB installer will begin. To view a detailed estimate of the time remaining to download, click the Purchases tab at the top of the Mac App Store window.
Once the download has completed an install wizard will appear. Click the Continue button to begin.
Read through the Software License Agreement and click the Agree button.
Click the Agree button again from the popup window to confirm your selection.
The installer will attempt to detect the drive to install to. If you want to install Mac OS X Lion on a different drive click the Show All Disks button.
To begin installation on the selected drive click the Install button.
After a few minutes of preparation the computer will reboot.
Once your computer has restarted the installation will begin. This process will take about 30 minutes.
You'll be informed when Mac OS X Lion is installed successfully and your computer will automatically reboot again.
Once your computer has restarted the second time you will be ready to go on your new installation of Mac OS X Lion.
A setup assistant window will appear to help you learn the new scrolling direction default in Lion. You can change this in the System Preference if you so desire.
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