You have few options to return the CERB payment

Online by CRA My Account

https://www.canada.ca/en/revenue-agency/services/e-services/e-services-individuals/account-individuals.html

By mail:

Mail a cheque or money order to the CRA:

1. Make the payment out to “Receiver General for Canada”
2. Indicate it is for “Repayment of CERB”
3. Indicate which eligibility period you are repaying
4. Include your Social Insurance Number (SIN) or your Temporary Tax Number (TTN)
5. Mail your payment to: Revenue Processing – Repayment of CERB
   Sudbury Tax Centre
   1050 Notre Dame Avenue
   Sudbury ON P3A 0C3

If you still have the CERB Cheque

You can return the cheque by mail.

Be sure to include:

1. the reason for returning the cheque (not entitled or overpayment)
2. your Social Insurance Number (SIN) or Temporary Tax Number (TTN)

Mail the cheque to:

Revenue Processing – Repayment of CERB
Sudbury Tax Centre
1050 Notre Dame Avenue
Sudbury ON P3A 0C3

You can check the following link for more information

Return or repay a payment: CERB with CRA – Canada.ca