You'll want to determine if the IRS has Accepted the information you already filed (e-filed). If they have, then you may need to amend your tax return.
If your return was Rejected, you can add the missing form or information, correct the reason for rejection and then resubmit again.
For additional information on how to Amend (Change or Correct) a Return You Already Filed please refer to the TurboTax FAQ below:
https://ttlc.intuit.com/replies/3288565