iOS 8 finally launched on September 17, and with it, the public release of Xcode 6. However, after upgrading you will notice that you can no longer compile using the iOS 7.1 base SDK. This is a problem for any app that isn’t yet compatible with the iOS 8 SDK. To see what SDK’s you have installed, open the Terminal app on your Mac and enter:
If you only see the iOS8 SDK folder listed, you will need to extract the iOS7 SDK folder from a copy of Xcode 5.1.1. Or you can just download it here. Copy your unzipped iPhoneOS7.1.sdk folder into the folder shown above. Shut down and restart Xcode. You will then be able to select from both SDK’s when building or running.
Remember that you will also need to download the iOS 7.1 simulator by opening Xcode and going to Xcode > Preferences > Downloads.