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, 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 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. You cannot e-file an amended return.  They have to be mailed, and it takes about 4 months for the IRS to process an amended return.

 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

Once a tax return has been e-filed it cannot be canceled, retrieved or changed.
If you have e-filed your tax return and need to change, add or delete information on the tax return then you will need to wait for the tax return to either be accepted or rejected by the IRS.

If the tax return is rejected then you can make your changes and re-file the tax return.

If the tax return is accepted then you will have to amend the tax return to make your changes. An amended tax return, Form 1040X, can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed. The IRS will take up to 4 months to process the amended tax return. Wait until you have received the tax refund or the taxes owed have been paid and processed by the IRS before amending the tax return.

On the Tax Home web page when the Form 1040X for amending a 2019 tax return becomes available there will be a link - Amend (change) return

ALL of your W-2’s and/or 1099’s were supposed to be on the same tax return.  Remember that next year and do not e-file until you have everything.  Now you have to prepare an amended return to add that W-2—and the process will take months.

https://ttlc.intuit.com/questions/2566895-what-does-it-mean-to-amend-a-return

How do I amend my 2019 TurboTax Online return?

A 1040X cannot be e-filed.  It must be printed and mailed.  It will take about 4 months for the IRS to process an amended return.  It is a good idea to use a mailing service that will track it, like UPS or certified mail, so you will know it was received.

Wait about 3 weeks from mailing and you can check the status of your amended return:

https://www.irs.gov/Filing/Individuals/Amended-Returns-(Form-1040-X)/Wheres-My-Amended-Return-1

https://www.irs.gov/filing/individuals/amended-returns-form-1040-x/amended-return-frequently-asked-q...

https://ttlc.intuit.com/questions/1908543-do-i-need-to-amend