https://stackoverflow.com/questions/18423896/is-it-possible-to-install-ios-6-sdk-on-xcode-5
Xcode 5 seems to remove all iOS SDKs, except for iOS 7. If you need to build an app for iOS 6 (and retain the iOS 6 UI & behaviour), then you'll need to re-download the iOS 6.1 SDK and symlink it into Xcode's SDK directory:
sudo ln -s iPhoneOS6.1.sdk/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.1.sdk
There are a couple of solutions in the linked Stack Overflow topic. This answer has a download link to the iOS 6.1 SDK, without having to re-download Xcode 4.