为了免下载安装Xcode，安装时使用了别人提供的Xcode.dmg安装，而非使用自己账号在AppStore下载的。
这样的安装模式会出现一个问题，更新Xcode时AppStroe会提示让你输入下载该Xcode的账号及密码，该账号不能直接修改。下面的方法可以删除原有账号信息，而后可以使用新的账号信息更新。
做如下步骤:
1.打开引用程序目录
2.找到Xcode，右键“显示包内容”
3.找到_MASReceipt文件夹
4.删除它，然后退出重启AppStroe，更新Xcode即可。
To make the AppStore forget the old credentials with which the app was downloaded, you must remove the _MASReceipt directory in the app itself.
To do this:
Open Finder and navigate to Applications,
Ctrl+Click XCode and choose "Show Package Contents",
Expand the Contents directory and click _MASReceipt to select it,
Type Command+Delete to delete the directory permanently---you will be prompted for your credentials since this is a protected file.
Quit and restart AppStore, then find XCode. The button should now say "Free" or "Install" instead of "Update". Clicking it will update your XCode to the latest version as the currently logged-in Apple us
原文:http://apple.stackexchange.com/questions/53648/mac-app-store-how-to-check-which-account-was-used-to-buy-app