I have an iPhone 4S model A1387. I want to update to ios 8. I currently have ios 7.1.2 (11D257).When I try to update either via iphone or itunes on my mac the response is "Your software is up to date." How do I get it to update to ios 8?

Sorry, but that is an iPhone 4, thus it can not be upgraded beyond 7.1.2

Force restart your iPhone.

Press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.

Then tap Settings > General > Software Update

If that doesn't help, make sure your Mac is running the latest version of iTunes.

You can do that by clicking your Apple  top left in your screen.

From the drop down menu click: App Store

After the App Store opens, select Updates from the menu top of the App Store window.

Please verify the model number at: Settings - General - About - Model = ??

Post back with results.

Correct, the A1387 is indeed a 4S, the model number MD200LL/A is not. It would appear that the phone has been modified/repaired and a 4S back has been installed.

You are welcome.