So you already submitted (actually transmitted) an efiled return? If you already efiled a 2020 return, there is nothing you can do until you find out if the IRS either accepts or rejects the efiled return. Unfortunately, due to late tax law changes the IRS will not open up to start processing until February 12, 2021, except for a few returns they pull early for testing purposes. In the meantime the return will sit on TurboTax servers with a "pending" status but cannot be retrieved to cancel or correct. You can check the status by logging into your Tax Home.
So unless yours is one of the few returns pulled early by the IRS, you won't know anything until February 12 or soon thereafter. Only then will you know how you will have to proceed.
If you have errors on your return or need to add something, you would be lucky if the IRS rejects your efiled return. If they reject it, that's the easiest to fix, since you can then simply correct everything, add the additional info, and resubmit it. If they accept it, however, then then only way to correct it is through a formal amended return Form 1040X.
So in the meantime do NOT make any changes in your return until you find out if it's accepted or rejected by the IRS. If you end up having to prepare an amended return, it has to start out exactly as it was when originally filed.
If you end up having to prepare an amended return, it's recommended that you wait on any refund to arrive first before amending.
Also, if you amend a Federal return, you will likely have to amend a state return, too, if applicable.