You can edit your Dependent info by selecting "My Info" on the left sidebar under the Tubotax Self-Employed logo. This will display the "Personal info summary", on this screen you should see your dependent name, select the blue Edit on the right and go through the screens for the dependent you need to make the change for. You will see the field to change the birthdate. Be sure to go thru the entire topic to make sure the change is complete.
Hope this helps!