You won't be able to make any changes until your return has been accepted or rejected by the IRS. If rejected, you would need to correct any information that caused your return to be rejected as well as make any other changes to your return that you see fit, then proceed to resubmit your return. If accepted, you would need to amend your return. You can follow the instructions in this FAQ here to amend your 2020 tax return.
You will need to amend if you already E-filed or mailed your return and:
If you need to amend your 2020 return you must wait until the IRS either accepts or rejects your return.
If the IRS rejects your return, you can make the changes and re-submit your return.
If the IRS accepts your return, you have to wait until it has been fully processed and you have received your refund. Then you can prepare an amended tax return and mail it in.