The iPhone 4S battery removal is a pretty easy process. Removing the back cover is super easy, so after that you just need to remove a couple more screws,then carefully lift out the battery. The iPhone 4S battery is a bit sensitive. If you puncture it in any way it will get bloated and ruined. So when unscrewing and prying be cautious.
This is a great video by iMobileDoctor. They pretty much run through everything you need to know in precise detail. Unlike most other smartphones, Apple has made the iPhone battery a bit more complicated to fix. Especially with the iPhone 3G and 3Gs. But with the iPhone 4 and 4S, it is simple as long as you have the right tools.