You can only ReFile returns for the tax year 2018 or later. For older returns, 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
For more information about changing your tax return, please see this TurboTax article: How to Change Your Tax Return AFTER You’ve Filed
