How do I fix a a calculation error on return already filed?

To correct a filed tax return you will have to amend the tax return.

An amended return, Form 1040X, 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 on amending a tax return originally completed and filed using the 2020 TurboTax editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/01/27439

Please explain what was the calculation error?  Maybe the IRS will adjust your return for you and you won't have to fix it.  Is it for Unemployment or the Stimulus Recovery Rebate Credit?