Should be simple, but surprisingly while you install the upgrade of xcode you are stuck on iTunes not closing properly.
Well in fact you can close it but xcode installer seems to miss this fact.
You ask yourself what's going on?
Does it sound familiar?
The reason is simple. Even though you close iTunes (BTW: why can't xcode installer do it by itself?) there is still iTunesHelper in background blocking Xcode installer.
So all you have to do is to open Terminal and issue:
$ ps -ef | grep iTunes
501 406 96 0 12:49AM ?? 0:00.14 /Applications/iTunes.app/Contents/MacOS/iTunesHelper.app/Contents/MacOS/iTunesHelper -psn_0_180268
$ kill 406
where 406 is pid of iTunesHelper
$ killall iTunesHelper
Have fun :)
PS: That's something new for Apple to have QA issues.