I can't help you with Quebec-specific information, but I do know that the easiest way to amend a previous year's tax return is through the CRA "My Account" feature. Setting up this account for the first time is a bit of a pain, as they insist on (snail) mailing you a confirmation code, rather than through email. Once your account is set up, it's fairly simple to go in and make updates. I only did this once a few years ago, so I can't remember the exact details.
