I have already filed but I put the wrong birth date for one of my dependants . How do I fix this?

[ToddL]

Once your return is filed and "Accepted", you can't make changes. If the return is Rejected, you can correct the error by deleting that child as a dependent and re-entering his/her information in the Personal Info interview.

The birthdate of dependent children is used to determine eligibility for certain exemptions and credits (e.g. a child must be age 12 (or younger)  to qualify for the Child Care Credit). 

Unless the error was big enough to create a problem with an exemption or a credit, it won't affect your return. Birthdates are not used for purposes of identification on e-filed or paper-filed returns.