In Xcode 6 , 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.
If the Xcode doesn't provide older version of simulator, the only way to do this is to install older version of Xcode.
download older Versions
Currently you can just use Xcode 5.x.
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