I was also running the same problem when I updated to xcode 5 it removed older sdk. But I taken the copy of older SDK from another computer and the same you can download from following link.
http://www.4shared.com/zip/NlPgsxz6/iPhoneOS61sdk.html
(www.4shared.com test account test@yopmail.com/test)
There are 2 ways to work with.
1) Unzip and paste this folder to /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs & restart the xcode.
But this might again removed by Xcode if you update xcode.
2) Another way is Unzip and paste where you want and go to /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs and create a symbolic link here, so that the SDK will remain same even if you update the Xcode.
Another change I made, Build Setting > Architectures > standard (not 64) so list all the versions of Deployment Target
No need to download the zip if you only wanted to change the deployment target.
Here are some screenshots.