Once you log into your CRA My Account with your user ID and password, you are directed to a page titled CRA Login and Security Options. From there, click on Change CRA Security Questions and Answers link.
On the following page, you will see the security questions that are currently active on your account. Select the question(s) that you wish to change, enter your password again, and click Next. You will be taken to a page where you are able to select a different security question, or to keep the existing question and update the answer. Clicking Next will confirm the change and show you the new summary of your questions.
Register for a CRA My Account.
Learn more about registering for a CRA My Account.