You cannot change or add anything on the return that you just e-filed, nor can you stop it.  It is too late, just like when you put an envelope in a US mailbox on the corner.

If you left out a W-2, a 1099G, or a dependent, or a 1099 etc…DO NOT change your return while it is “pending.”  The changes will go nowhere.

Now you have to wait until the IRS either rejects or accepts your return.  If your return is rejected, you will be able to go into your account and make the necessary changes to your tax return and re-submit your return. 

 If the IRS accepts your return, however, then you have to wait longer until it has been fully processed and you have received your refund.  THEN you can prepare an amended tax return and e-file or mail  it in. You have to be able to work from that return exactly the way it was when it was e-filed originally.  You will need to use a form called a 1040X.

  Meanwhile, DO NOT go in and start changing anything on your return in the system, or you will make a mess for yourself.  Sit tight and wait until you see what the IRS does with the return you just e-filed

You cannot amend a tax return that was just accepted.   You have to wait for it to be processed by the IRS.  Trying to send in an amended return while they are still processing the original return could confuse things and delay your refund for months.

Do not try to change anything on your return now.  When you amend, your Form 1040X has to be prepared from the return exactly the way it looked when you filed it originally.   You have to wait.   Once your return has been processed you will complete a Form 1040X   --- which is not yet available --- and submit it to amend the mistakes made when you filed your original return. It takes the IRS at least four months to process an amended return.

The 2020 Form 1040X does not become available until the end of March 2021 (subject to change).

https://www.irs.gov/forms-pubs/new-information-for-form-1040-x-filers