You can amend your return when you discover any mistakes or omission on your original filed return which resulted in either an increase or decrease in the amount of tax payable.

CRA Preferred method

Write a letter/message to the Tax Center where you filed your tax return with the changes in the tax return along with explanation and documents as T3, T4A, T3, T5 etc. The CRA also prefers that you include the T1-ADJ form along with the documents and an explanation.

Alternatively, file the adjustment through my CRA account for the current year and the previous 10 years.