Run app in iOS 6 Simulator in Xcode 6
In Xcode 6 beta 3, iOS simulator for iOS version 6.x seems to be deprecated.
Apple deprecates old technologies very aggressively. Formaly saying, you should deprecate support for older version of OSes such as 6.x.
Anyway in practice, we have situation that we can't do it. If the Xcode doesn't provide older version of simulator, then the only regular way to do this is installing older version of Xcode.
Install Xcode 5 and run its simulator manually. ([Xcode] -> [Open Developer Tools] -> [iOS Simulator])
Each Xcode versions can be installed and run side-by-side. See here to know how to get old Xcode versions.
Currently you can just use Xcode 5.x.