Have you filed the return yet? If you have, this FAQ has more information on changing a filed return: https://turbotax.community.intuit.ca/community/tax-return-amendment/help/i-have-filed-my-tax-return-can-i-still-make-changes/00/873608
If you haven't filed yet, and you did your return separately from your partner, you can click on My Info > Marital Status and correct your status. If you prepared your return together with your partner, then you'll both need to start your returns over, as you can't switch to single after the coupled return has been started.