You won't be able to change your marital status if you have already filed your return. You would have to use the Canada Revenue Agency (CRA)’s My Account service or send in a request by mail, using a completed Form T1-ADJ, T1 Adjustment Request.
You can log into the CRA’s My Account service on their website: My Account for Individuals
You can download the T1ADJ from this CRA webpage: T1-ADJ T1 Adjustment Request