You will need to amend the 2018 tax return that you filed and change your dependency in the My Info section for your name.

See this TurboTax support FAQ for amending a 2018 tax return - https://ttlc.intuit.com/questions/4667233-how-do-i-amend-a-2018-return-in-turbotax

Was your return Accepted?  If you claimed yourself and your parents return rejected they will have to mail their return this year.  And you have to amend.

Or did your return reject?  Why do you want them to claim you?  Are you under 19?  Or made less than $4,150?