How to turn service off on a iphone 4s?

I'm buying an iphone 4s from my friend and it still has his data and everything of the phone. He can't cancel his plan and it still says it has service on the top left part of the screen. His bought a iphone 5 from another friend and he just put his sim card in the 5 and it works perfectly how can i do the same thing but with a 4s?

If you don't have a sim already you can buy them online activate it and put it in the sim slot on the Iphone 4. I did this with mine last month.