If you are using the TurboTax online editions you can change your filing status in the My Info section of the program.
Click on My Info on the left side of the online program screen. Edit your personal information. If showing Complete for your name, click on your name and click on Revisit
If you are using the TurboTax desktop editions you can change your filing status in the Personal Info section of the program. On Your Filing Status, click on Update