To do an adjustment, you just need to make a copy of you return (for your records) .  Tnen make the changes, and fill out form T1REFILE (Date, Amended checkbox and description) The you EFILE as a normal return.

WE have instructions here : https://profile.intuit.ca/support/en-ca/help-article/tax-return/refile-t1-return/L5sSaq908_CA_en_CA