I filed my taxes already but claimed my girlfriend as a dependent not knowing it wouldnt allow her to file her taxes I dont know how to change it.

If the return was already filed, you will need to wait until it has been processed and you have received your refund or confirmation that any tax due was paid. (If you file an amended return while your first return is being processed, it can cause extended delays for both returns)

If the IRS would reject your return for any reason, then you would be able to go in and make any necessary changes to your tax return and resubmit.

How do I fix a rejected return?

How do I amend my 2019 TurboTax Online return?

Amended returns can only be mailed. It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS receives it.

You can check the status of the amended return here, but allow 3 weeks after mailing.

How do I track my amend status?

To make changes before filing:

1.    Sign In to TurboTax Online

2.    Click "My TurboTax" menu

3.    Press "Visit My Tax Timeline" 

4.    Under the "SOME THINGS YOU CAN DO" heading, choose "Add a state" (you're not really adding a state, this allows you to get back into your return). 

5.    You may now make the corrections to your tax return.