Apple offers to let you trade in your old iPhone and offers credit towards the purchase of a new phone or (when it is worthless) passes it on to reputable smartphone recyclers. To make the process easier still, the company can send you a pre-paid trade-in kit or a shipping label which simplifies the process of sending your phone. You can also do it in person at an Apple store.
Apple asks you to answer several questions when performing an iPhone buyback that ask you to provide details of the model, brand and its current condition. They will then give you an estimated trade-in value for the phone or provide a convenient way to recycle it.
If you accept the trade-in estimate that Apple offers they will give you instant credit to the value of the trade-in that you can use to make another purchase or will provide you with a gift card that you can use at any time.