If you have recently updated to the latest version of XCode you might be wondering how to get your 6.1 iPhone SDK back. The new XCode versions do not come with previous iPhone SDK’s so we need to obtain them from older XCode (download them from Apple) and symbolic link them to the right place. First, copy the SDK’s from the download and place them somewhere which wouldn’t get replaced on future XCode upgrades. For my case I left them in /SDKs/iPhoneOS.platform/Developer/SDKs. Next, show package contents of your current XCode.app and navigate to Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs, and make the link with this:
sudo ln -s /SDKs/iPhoneOS.platform/Developer/SDKs/iPhoneOS6.1.sdk iPhoneOS6.1.sdk
Just remember to redo the links whenever you update XCodes.