If I have the Iphone 5 can I get the?

So I have the iphone 5 right now, been having it for a while. I'm with tmobile and I just gave a down payment and I'm paying it monthly(I'm not done paying it) if I want to get the iphone 5s the same way when it releases. Would I have to pay the 5 off first or can I pay both off at the same time? Please answer only if you know.

You would have to pay it off first.