How to remove an application update file?
You can try these steps:


Open Launchpad, press and hold the Option key. If you see a cross icon towards the top left side of the app (Xcode) icon, click on it.
Make sure that Xcode is really removed by checking for its absence in /Applications directory. Then restart your computer.


Either one of the two steps should remove the icon from the Launchpad. You can install Xcode afresh thereafter.