If you still haven't submitted your return for e-file yet, open TurboTax and search for stimulus, and select the jump to stimulus link at the top of the search results.  This will return you to the relevant section in TurboTax, where you can enter your updated info.

If you’ve submitted your return since you posted this question, please see the TurboTax Help article I've already claimed the Recovery Rebate Credit. How can I review and update my 2020 tax return? or I didn't claim the Recovery Rebate Credit. How do I review and update my 2020 tax return? as appropriate, for guidance.

The IRS requires you to report all relevant stimulus information when you’re filing your return.

I had similar problem. Did not make an entry for EIP2 and submitted the return. When I found the payment I started  an amendment. When I entered the correct EIP2 amount, my refund was significantly reduced to the point where it says I owe the IRS money. To find the place in turbo Tax to make these EIP entries,  Go to "Federal Taxes", then to "Federal Review." I'm not convinced Turbo Tax is handling these inputs properly. I'm stuck now, trying to figure out what to do next.

I luckily realized that I had saved a draft copy of my return so I threw away that other copy and reverted back and was able to correct my EIP payment info.

You do not need to amend for the EIP corrections.  The IRS will make the adjustments and notify taxpayers of those changes.  They will send a letter.  See questions G-8 through G-11 in this link for more information on what to expect.