I have already downloaded the new Xcode 5.0, but I wanted to build an app for iOS 6...
How can I do that from Xcode 5.0? Or do I need to have a version of Xcode 4 installed so I can do that?
Xcode does let me change the deployment target as the screen below:
But it doesn't let me use a older Base SDK
Just set the Deployment Target to iOS6, in the configuration of the project:
First of all you need to download the older Xcode you want to build for.
After that, you copy the .sdk in the folder of the older Xcode /Applications/Xcode/Contents/Developer/Platforms/iPhoneOS.platform/platform/SDK
and paste it in the same folder, but now, from the Xcode 5.0
Now, you should be able to choose between all the .sdk installed on that folder under the "Build settings" tab of your project, under Base SDK.
来源：https://stackoverflow.com/questions/18917740/how-can-i-build-for-ios-6-with-xcode-5