I recently preordered an iPhone 6s and my parents both got an iPhone 6. Our contract was supposed to be up in March of next year, but Verizon let us upgrade early if we switched to a device payment plan, which we did. However, I'm not entirely sure on whether or not we need to send our old devices back, and if so, how would we do that? I believe it was mentioned somewhere, but I can't find information on sending your device back due to early upgrade. My Verizon is less than helpful.
Typically you have to send your current device back and return information and shipping labels should be included in the box with your new devices.
Pay close attention to the return label details and instructions. If you're turning in an iPhone be sure to turn off "Find My iPhone" and wipe your info from it. Make a copy of your shipping label so you can track your return and have some sort of proof of delivery once it reaches its destination.