If you have already e-filed or mailed the return, you will need to wait until the IRS has accepted or rejected that return before you can make any changes to add new tax documents or information.
- If they reject it, you just make the needed changes and re-submit the return.
- If they accept it, you will need to wait until you have received your refund and then amend that return to make changes, if needed. Amended returns have to be printed and mailed in and take up to 16 weeks to be processed.
See some reasons you'd need to amend your return below
https://ttlc.intuit.com/replies/3318732
