they should have given you a prepaid return label in the box.
If you didn't get one, do chat support and they will send you one in the mail, or simply take it to any UPS and they have a corporate agreement with verizon and will print you up a return label.
Make sure to get a receipt.
I had the same issue. Received a new router today with no return label...however, at the bottom of my packing slip showing what they shipped was a small note stating the router I was to return was an older model they no longer supported and was not required for return. May be same issue. I would check the packing slip and contact customer service either way.