Once a return has been submitted electronically, it cannot be stopped and will be transmitted to the IRS and processed. You will receive an acknowledgement from the IRS indicating the acceptance or rejection of your return, generally within 48 hours after they receive it.
If the IRS rejects the return, you can make necessary corrections and resubmit the return electronically for no additional charge (assuming you e-file through the same TaxAct product from which you initiated the return).
If the IRS accepts your return, you will need to file Form 1040-X Amended U.S. Individual Income Tax Return, to make any changes (form 1040-X for tax years before 2020 cannot be e-filed, so they need to be mailed).
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.