How do I use a showroom model of an iPhone 4S?
I would suggest doing a DFU restore. You can do this by:


Plug your iPhone into a computer running the latest version of iTunes.
Hold the home button and the sleep/wake button for exactly 8 seconds.
After this 8 seconds is up, release the sleep/wake button, but continue to hold the home button until iTunes recognizes your iPhone.
Then, choose the Restore option (this should be the only option) and wait for iTunes to download the necessary software and restore your iPhone.