Q: my iphone 4S is on iOS 7.1.2, and shows that the software is upto date.  How do i upgrade that to iOS 8 and above as i need to download an App that requires that?

If its really a 4S is should be able to update to iOS 8.

However if its saying its up to date, its more likely to be a 4 (not s) and iOS 7.1.2 is as far as it can go.

You may however still be able to get compatible versions of Apps for it though. In iTunes on your computer, download the App you want a compatible version for using the same Apple Id that the iPhone has for the App Store. For Paid Apps its best to consult with the App maker to make sure there is a compatible App. With the App Downloaded to iTunes on a computer you can go into The App Store App on your iPhone, tap on Updates, then tap on Purchased, and Look for the App there. Tap on it to begin the download. It will likely say its not compatible, but if there is a compatible version it will offer to download it.

Install the latest compatible version of an app on an earlier version of iOS or OS X - Apple Support