Just figured it would be worth mentioning that technically it isn't an exchange unless the phones are of equal value (because then it could be done in a single transaction).
Otherwise the phone is returned to a gift card then that card is used to purchase the new phone.
If for example you wanted to return a 6 plus for a lower value 6 and wanted the difference back in your account then you can return to your credit card, just note then that the refund may not be instantaneous, and there may be a slight delay depending on your bank.
A lot of waffle that probably won't apply to you OP, but the last bit is important for those who don't have a big credit limit and want to purchase their replacement phone right away.