The iPhone has a 12 month warranty. It could be replaced under warranty. To be upgraded under NEXT, you must be at the upgrade time of your NEXT agreement. It depends on which plan you are on.
AT&T Next 24 then divides the regular price of your smartphone into 30 low monthly installments.
Trade in your qualified smartphone to upgrade after 24 installments. Your payments are conveniently billed on your monthly statement.
Other Options:
AT&T Next 18: Divided into 24 installments; trade in and upgrade after 18 installments.
AT&T Next 12: Divided into 20 installments; trade in and upgrade after 12 installments.
But your phone must be in good physical and fully funcional condition.