If you purchased Lightroom 4 from the Mac App Store and want to upgrade to Lightroom 5, purchase the upgrade from Adobe or another vendor. (For example, get the upgrade from Amazon, Best Buy, B&H, and so on).
Install the Lightroom 5 upgrade
- Make sure that the Mac App Store version of Lightroom 4 is on your computer. It provides validation required for the update installer to authenticate.
- Run the Lightroom 5 installer and follow the onscreen directions. The Lightroom 5 upgrade process detects the Mac App Store version of Lightroom 4 installed on your computer, and requests the upgrade serial number.
After Lightroom 5 is installed, you can uninstall Lightroom 4. See this TechNote for assistance in uninstalling Lightroom.