You can only file one tax return per tax year. If you submitted your return to CRA already, any changes must be made by an adjustment. If you're a CRA My Account holder, you can make these changes online in your CRA My Account using the "Change a return" option. This is the easiest way to make an adjustment.
If you're not a CRA My Account holder, you can submit an adjustment by completing a T1ADJ form and submitting it by mail.
It's faster to just set up a CRA My Account. The registration process is much quicker than the time it takes for CRA to process a paper adjustment.
https://turbotax.community.intuit.ca/replies/3723126
