Did you already file it as Single? If you haven't filed it yet you can go back yo My Info and click Edit by your name and change it. Say you are married and not filing together.
If you did file and it was Accepted you need to send in an Amended return to change it. You should not amend until the first return has fully processed and you get the first refund or your tax due payment has cleared.
How to Amend this year
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...
