How to sell my contract iPhone?

I have an iPhone 4S and am wanting to sell it to get a 5s, but I'm still paying my phone off as I got it for free, how could I sell it? Also how would I be able to take my sim out and still have apps on the phone itself?

The phone is yours, the contract is the air time/data/texts you use. You can sell the phone and get a new one anytime you want.

Yes, as "NoName" said, the phone is yours, but be sure to call your provider to inform them of your intentions in case there are stipulations.