Apple do not support two versions of iOS back. They expect you to upgrade to a newer, slower version of iOS every time it's released. You won't be able to get

Apple do not support two versions of iOS back. They expect you to upgrade to a newer, slower version of iOS every time it's released. You won't be able to get the new iOS 6.1.6 on your iPhone unless it supports iOS 6 but not iOS 7, it is basically only for iPod 4th gen.