Turbotax said I received the correct amount for my first and second stimulus checks but i was shorted on both payments how do i change my taxes if i filed already?

Is the dependent, whose stimulus payment you are missing, listed as a dependent on your 2020 income tax return?  Is the dependent a qualifying child?

A qualifying child is a child who meets the following conditions:

• Relationship to the individual who’s eligible for the payment: The child is your son, daughter, stepchild, eligible foster child, brother, sister, stepbrother, stepsister, half-brother, half-sister, or a descendant of any of them (for example, grandchild, niece, or nephew).
• Child's age: The child was under age 17 on December 31, 2019.
• Dependent of the individual who's eligible for the payment: The child was claimed as your dependent on your 2019 tax return or in the Non-Filers: Enter Payment Info Here tool.
• Child's citizenship: The child’s a U.S. citizen, U.S. national, or U.S. resident alien.
• Child's residency: The child lived with you for more than half of 2019.
• Support for child: The child didn’t provide over half of their own support for 2019.
• Child's tax return: The child doesn’t file a joint return for the year (or files it only to claim a refund of withheld income tax or estimated tax paid).

The payment will include $600 for each qualifying child listed on your 2019 tax return with a valid SSN or ATIN. 

The TurboTax software adds together the entries that you made for receiving stimulus check 1 and stimulus check 2.

Then the software computes the amount of your stimulus check amounts based upon the information that has been entered into the tax software. 

If you are due an additional amount, it will be issued as a Recovery Rebate Credit (RRC) on line 30 of the 2020 1040 tax return.

If you believe that the Rebate Recovery Credit (RRC) on line 30 of the 2020 1040 tax return is not correct, then you should: 

• Check the amounts entered for stimulus check 1 and stimulus check 2, and
• Make sure that you have correctly entered your dependent information.  

If there is a difference, it is in one or both of these situations. 

In TurboTax Online, you are prompted to input stimulus check 1 and stimulus check 2 information under Review down the left hand side of the screen.

You can also access your stimulus check choices by following these steps: 

• Down the left side of the screen, click on Federal.
• Across the top of the screen, click on Other Tax Situations.
• At the screen Let’s keep going to wrap up.  Click on Let’s keep going.
• At the screen Let’s make sure you got the right stimulus amount, click Continue.
• At the screen Did you get a stimulus payment? you can update your stimulus check entries.  

See also this TurboTax Help.