You can do this via the Swap option as you will be given a value for your iphone 13, if that value is enough to pay off what you owe against it then you will have no more to pay against it. If itisn'tenough then the difference will be added to your next available bill and if is more than what you owe then the difference will added to your bill as a credit that will be used against a future bill.
You should also beware that there are long wait times for the iphone 16.
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