You will give the CRA at least 48 hours (sometimes longer) for the reception and processing of your tax return. Once that is done, you will log into your MyCRA account (link below), open your already filed tax return and there will be options to make changes to the already filed return.
Log in to MyCRA: https://www.canada.ca/en/revenue-agency/services/e-services/e-services-individuals/mycra.html
