How to uninstall an uninstalled app from the App Store? I tried to install Xcode from the App Store. While it was being downloaded, a network problem occurred so the installation didn't complete. However, in the App Store, it appears marked as installed (so now I can't install it).

I don't see any uninstall option. Is there any way to uninstall it from the backend or something like that?
In the special case of Xcode, the App Store downloads an installer app that you use to install Xcode. Look in /Applications for "Install Xcode.app" and delete that. 

When you get Xcode installed properly, if you delete the installer, the App Store won't think it's installed and won't display updates.