I hit the problem with my Mac Book Mavericks build that the App Store wouldn’t run an Xcode Upgrade. It offers me the terms and conditions but there’s no continue button. This thread on the Mac support forums offers a very complex diagnostic process or proposes that you download a free application to ‘blow it through’. Having done this, ask for the Xcode upgrade using the App Store. It seems that it sets an environment variable or writes a cookie so that it knows you’ve signed in and agreed the new ToS and doesn’t offer the revised terms and conditions when you press the update button. Worked for me.