The stimulus payments are not considered income and do not have to be included in your return.  If you did include the payment in your 2019 return, you can amend it.  

Please see the instructions contained in these TurboTax Help articles to amend your return: 

• How to amend (change or correct) a return you already filed
• How do I amend my 2019 TurboTax Online return?

How could you have reported the stimulus money on a 2019 return---there was no place to enter it in the 2019 software or on a 2019 tax form. 

You report it in the Recovery Rebate Credit part of the 2020 tax return in the Federal Review under Other Tax Situations.