Welcome to the Fido community!
You will need to reach out to customer support to process the cancellation.
You can contact them through any of the available methods listed here even from outside of the country.
If your account will have a credit after you cancel it Fido will send you a refund via cheque 3-6 weeks after your final invoice has been produced.
If you will not be returning to Canada let them know so they can update the account with your international address so they can mail the refund cheque there. If you made your payments via credit card you can also request a refund there.