Basically they'll process the return of the 6S+ and unwind the contract. Then they'll sell you a 6S with a new contract. IF you are within the return period.
If you have gone over the return period:
"You can upgrade anytime after six months, as long as you’ve made the equivalent of at least 12 payments plus any taxes and fees paid at the time of purchase. You’re free to make payments at any time."