Filing as a dependent

Did you e-file yet?  If you e-filed and your return was accepted, then when your parents try to e-file and claim you, their e-file will be rejected.  They will not be able to e-file at all this year.  They will have to file by mail.  And yes, you will have to amend.  But even if and when you amend, it will not make it possible for your parents to e-file, since your amended return will never catch up in the system this year.   You need to give them a heads up so they know now that that they cannot e-file.  

Your mistake is a common one made by people in your age group who do not understand how you are going to affect your parents' return.   Sadly----the mistake makes it take a lot longer for them to get their tax refund.

AFTER your return has been fully processed, then you should amend it and change the way you answered the question about whether you can be claimed as a dependent.  And you will probably have to pay back some of the refund you received incorrectly especially if they send you stimulus money you are not entitled to.

If you did not mark on your tax return that you can be claimed as a dependent by someone else and you have already e-filed your return, then your parents cannot e-file their tax return and claim you as a dependent. They would have to file by mail to claim you as a dependent.

As for you, you need to amend your own tax return and indicate that you can be claimed as a dependent by someone else.

Please see this TurboTax FAQ for instructions on how to amend your tax return in TurboTax.