Unable to update to Xcode6 from AppStore
I've had this happen with other apps in the past and what works for me is to:


kick off a Time Machine backup (or equivalent)
delete the app using Launchpad (or deleting from from /Applications)
restart


The restart is probably not needed in the vast majority of cases, but it ensures that no files from the app are still open from any process still running.

At that point, the app store should have a good shot to install the app (Xcode in your case).