As a general rule of thumb, it’s best to stick with the latest release of Xcode. That said, there are times when running an older release can be a good thing.
You can download older versions of Xcode by logging into Apple Developer Connection – Downloads and ADC Program Assests (connect.apple.com) – You will need to be a member of the developer program to login.
Once on the site, from the Downloads section, choose Developer Tools – you will be shown with a list of all Xcode versions, back to 1.0 released in October of 2004.