If you have filed your 2021 tax return it cannot be changed. You would have to amend the 2021 tax return to make changes.
An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 16 weeks or longer to process an amended tax return.
Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.
See this TurboTax support FAQ for amending a 2021 return - https://ttlc.intuit.com/community/amending/help/can-i-e-file-my-1040x-to-amend-my-return/00/26008
Note - If you indicated that you did Not receive the 3rd stimulus payment on the filed tax return and now you know that you actually did receive it, there is no reason to amend.
The IRS knows that the 3rd stimulus payment was sent to you so they will adjust your tax return accordingly.
