Turbo tax cautions you that once you submit it's like dropping it into the mailbox.
You can file a "corrected return" as long as it is mailed before 4/15/2021. You have to do it on paper, so you should be able print your correct return through turbo tax. Print it and then write "corrected Return" in red at the top of the 1040. Then mail it in based on the information in the instructions in the 1040. Always send certified return receipt.
I will say that depending on what the error is you may not have to do anything. For example if you left off a W-2 then the IRS system will catch that error and adjust your electronically filed return and adjust your tax. If that is the case, then you don't really have to do anything.
But if you want to change your return, if you file after 4/15/2021 then you will have to use a form 1040X which is a pain but will be available to do online using the software from turbo tax.
Amended Return Frequently Asked Questions | Internal Revenue Service (irs.gov)
