Disabling Find My iPhone on a broken device?

I had an old Apple iPhone that suddenly stopped working. The screen would not turn on at all. I am insured so I returned it to Verizon in exchange for a replacement phone. Now, I keep getting a message that I returned my old phone with Find My iPhone active and I need to disable Find My iPhone or risk losing my trade-in credit. How do I disable Find My iPh0one on the old device.