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.
When doing an exchange of an iPad or iPhone at the Apple Store, regardless of value, the refund is done onto a gift card (unless it's a swap for something of lower value in which case the difference is refunded and the rest is put onto a gift card). You cannot return and sell an iPhone/iPad in the same 'transaction', they have to be done separately for some reason - something to do with the IMEI/Activation server.