How to amend (change or correct) a return you already filed

https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...

You will change the answer about how much you got in advance for the stimulus payment to zero advance payment. 

To get to the stimulus questions in TurboTax at any time, you can search for stimulus or recovery rebate credit then hit ENTER and there should be a Jump to link

If you are trying to trigger it,
Click on Other Tax Situations at top
Click on Let's get started
Answer some questions then Continue
Then click on Let's keep going
Next screen is for the Third Stimulus, if you qualify