Hello @hyojung,
Welcome to the community!
You would need to contact customer service directly and let them know that you will be leaving the country and the exact date you would like to end services.
You should note that if you are currently financing a device you will need to pay off the remaining balance of that device, if not then you are okay. If the cancellation is done during your billing cycle it is possible you will receive a credit when your final invoice is produced which gets refunded by cheque to the address on file. Since you would be leaving the country, if you make your payments by card then you can notify the agent to have the refund be put back to the payment card, not by cheque.
