If you haven't filed your return, you can change your filing status by navigating to "Personal Info" on the Tax Timeline.
In the "Personal Info Summary", you can click "Edit" next to your filing status to update. (screenshot below)
You can then select "No" to the question "Do you want to file this return together with your spouse?" then click "Continue" to save your answer and complete the interview process.
Please note that many tax breaks will be removed for Married Filing Separate filers, see this link for more information: https://ttlc.intuit.com/replies/3288477
