When to amend your T1 Return?
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’s 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 such as T3, T4A, T3, T5 etc. 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 last 10 years.