I have entered my old SIN on my tax return on turbo tax. How do i change my return

If you have not NETFILEd your return yet, then you can go back and change by selecting My Info>[Your Name]’s Personal Information in the left side menu.

If you have already filed your return, please contact the CRA. If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are somewhere else you can call 613-940-8495. Here are more ways to contact the CRA: https://www.canada.ca/en/revenue-agency/corporate/contact-information/telephone-numbers.html