You can make changes to a filed tax return by filing an adjustment. The CRA requires that you wait until you’ve received your Notice of Assessment (NOA) before filing an adjustment. In many cases, the CRA will catch the mismatch and automatically adjust your return for you.
How you file your adjustment depends on how your original tax return was filed, and whether it was a federal or Québec return. Instructions on filing adjustments for both scenarios are provided here: