Ending iPhone Upgrade Program

It seems the answer to that may be no. See this, iPhone Upgrade Program Terms and Conditions

If you do not exercise your upgrade option, it would appear you must keep the phone and pay the balance. Even looking at #9, you can turn in the phone if desired, but you are still responsible for the balance of the financing, cannot exercise your upgrade, and end up paying the difference.

I'm not understanding your problem. You are leasing a phone and the purpose of the program was for an early upgrade, or to keep the phone and pay it monthly. If you no longer what the device, you can check to see if you can turn it in, but I'm just saying that my particular interpretation of the terms is that you have to pay it off. To be absolutely sure, contact Apple, or stop by the nearest Apple Store and ask them. If you did not intend to use the program as designed, why did you enter into it. You should have purchased the phone for full price, and then you could have sold it if you were not interested in keeping it at any time during your ownership. I'm afraid you may have misunderstood the terms of the program when you signed up.