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The Canada Revenue Agency (CRA) lets you make changes to tax returns submitted in any of the previous 10 calendar years. You can make changes to tax returns you have already filed online or by mail. Changes to your tax return are only allowed in certain situations, and you must include supporting documentation with your request.
Please visit our TurboTax article on How to Change a Tax Return and I have filed my tax return. Can I still make changes? to learn how to change your tax return.