How can I go back to iOS 6 on iPhone 4S for testing the applications that I have developed?

I am working in a software company and we develop iOS applications for our clients. Last weekend I borrowed an iPhone 4S from one of the fellow developers and updated it from iOS 6 to iOS7 while testing.

Later I realised that he needs iOS 6 on this device to be able to test the applications that we develop. It is to ensure that the application is running smoothly on iOS6 as well because some of the clients have this requirement.

I tried restoring the iPhone but it is only allowing me to restore to iOS7.

I understand that we have an option in Xcode to test the developed applications on simulator but we need to test certain things on device as well. Please help in learning how can this be done ASAP.