TurboTax Online is not a good vehicle for this sort of thing. You don't want to modify the tax return that you actually filed. Any changes in TurboTax Online are saved automatically and permanently. There is no backout or undo, and no way to exit without saving.
The best approach would be to buy a copy of the CD/Download TurboTax software and install it on your Windows or Mac computer. You could then download the .tax2023 data file for your tax return from TurboTax Online, open it in the CD/Download software, and save a copy with a different name. Then add the income that was omitted and see what happens to the taxable Social Security. You would be able to look at the Social Security Benefits Worksheet in forms mode, or create a PDF of it.
If you filed a state tax return, you have to install the state software before you can open the .tax2023 file. So you would want the Deluxe or higher software, which includes one state.
You could try calling TurboTax support on Monday, explain the circumstances, and see if they will give you a free download of the software.
