Are you running the Gold Master version of Lion? If so, you’ve probably noticed that you can’t update to the retail version of Lion in the Mac App Store: but you error that says a newer version of the software is currently installed.
Luckily, there’s an easy trick to getting retail Lion to install over Lion GM.
Here’s what you do. Open the Mac App Store and click on your ‘Purchased’ tab. You should see ‘OS X Lion’ in this list.
Click the text link reading ‘OS X Lion’ while holding down the Option key. You will now be whisked away to the OS X Lion page. Now when you click the ‘Install’ button, the installer will download as promised!