You can only file one tax return on each account using the online product. A new account must be created for each person filing a tax return.
Each tax filer can only file one return. If you left information off your original return, you can't file another return to enter it. You must prepare, print, sign and mail an amended return form 1040X to make any changes to your original return that has been filed and accepted. Post back if you are in this situation for more information about amending a return.
Do you mean you got another tax form like a W2 or 1099?
You cannot 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. If your return is rejected, you will be able to go into your account and add or change it and resend it.
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