CHALLENGE - Want to get an app that needs iOS8.0 (currently iOS7.1.2) and dont wanna go to 9.2.1

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How do I get an app that needs iOS8.0 (that's the message i get when i try to DL it) on an old iPhone 4s.

My old iPhone 4s (not JB) is still with iOS7.1.2.

I do not want to get iOS 9.2.1 (or later version) on my old iPhone 4s.

Thanks a bunch.