how can i return the router?

[#1]

i will leave USA next month, how can i return the router for web?

[#2]

Suggest In Person to a VZ store

Get a receipt

[#3]

If their is no store near to you, you need to contact Verizon 1-800-VERIZON and ask for an authorized return.  They will send you a box and labels.  I believe they are still using ups, but whatever company they are using make sure to get a receipt.

Don't tell them to far in advance that you want to turn off service, they have a tendency to do it immediately rather than when  you want it.