如果电脑上安装了几个Xcode版本，如Xcode5.1.1和Xcode 6 beta 1，这是项目都会默认用Xcode 6 beta 1打开。在简介里面修改打开方式也无济于事。到网上找到如下方法，亲测可行。
To modify files association for certain app one can use lsregister tool. So to re-register the app there is -f parameter, and to unregister -u. Everything can be in fact done with just one command:
$ lsregister -f /Applications/Xcode.app
Or if you want to unregister Xcode 5 and not touch manually set associations earlier:
$ lsregister -u /Applications/Xcode5-DP5.app
lsregister is located in /System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/, so add it to your $PATH
Re-login to OS X for changes to apply.