My question title probably sounds confusing, so allow me to explain...
Let's say you have an iPhone 3GS that's still running iOS 5. Is it possible, today, to upgrade that device to iOS 6 (the latest supported version)? Or is it now stuck on iOS 5 because the latest version, iOS 7, doesn't support the 3GS?
Put another way, if you were to go to Software Update on that device, would it offer you an update to iOS 6, or would it say nothing is available because iOS 7 isn't compatible?