How do I change a return that's already been assessed?

You have to wait until your return has been processed before you can make any changes. You can only make changes to returns going back 10 years. 

Federal income tax returns

Once you've filed your income tax return, you have to wait until you’ve received your notice of assessment from the Canada Revenue Agency (CRA) if you need to make any changes.

There are four ways that you can make changes to your federal tax return, after you have received your notice of assessment:

Note: If this is your first year filing a tax return, you won’t be able to use the services typically available through the CRA My Account; this is because the CRA requires that you file one tax return before you can register for their My Account service.

Québec income tax returns

There are two ways that you can make changes to your Québec tax return:

Where can I learn more?