If you missed entering something in your tax return and have not filed it- You will go back to the section of the return where you can edit or delete an amount missing.
If you have already filed your return you will need to wait until you receive your notice of assessment in the mail to verify that CRA never caught the error and fixed it. If not then you will be able to ReFile your return.
If you need more help you can contact our customer service for assistance.