If you have already e-filed or mailed the return, then you will need to wait until the IRS has accepted or rejected that return before you can make any changes.
If they reject it, then you can make the needed changes and re-submit the return.
If they accept it, then you will need to wait until you have received your refund, if applicable, 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.
If accepted, wait until you have received your refund, then you can go here and get instructions for amending that return.