Click on Personal Info
Edit Your Name
Continue to the screen Do Any of These Apply to (Your Name)?
Uncheck the box labeled Someone else can claim (Your Name) on their tax return and continue with the section until it completes.
See Screenshot
If you filed your return and it was accepted then you have to amend the 2018 tax return to change your dependency.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/questions/4667229-how-do-i-amend-my-2018-return-in-the-turbotax-cd-download-...
