If a new bill passes and authorizes an additional stimulus payment, it won't be reported on 2020 tax returns. It will, in all likelihood be an advance payment of a credit on 2021 tax returns that you would file this time next year.
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No. You cannot cancel or change or add anything on the return that you e-filed. It is too late, just like when you put an envelope in a US mailbox
Now you have to wait until the IRS either rejects or accepts your return. Which won't be until Feb 12 this year. If your return is rejected, you will be able to go into your account and add or change it and resend it. Click on Add a State to let you back into your return..
If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund or paid the tax due. THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return. Meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.
You can check your efiling status here,
How to amend an Online return