How to update xcode via app store?

I just update xcode from 4.2 to 4.2.1 via App Store. However, after downloaded the latest version of xcode, there is nothing happens!
I searched the internet, and finally found that I must run /Applications/install xcode.app.... it means App Store only download the install package, and place it into /Applications, but doesn't run it for you.

You can click "install xcode" from launchpad, and follow the instructions.