Start an amended return. Make your changes and then cancel the amendment when you are finished seeing the results.
That's risky, and I wouldn't recommend that anyone experiment in their original return after filing. Online TurboTax saves the entries after every screen. While it is possible to cancel the amended return's Form 1040X, any entries that were changed during experimentation would have to be manually changed back, which could get rather involved. Canceling an Online amended return gets rid of the Form 1040X but does not automatically revert the changed return back to the original as-filed copy.
While I don't recommend doing that, you especially should not do it without at least first downloading for your records the original as-filed PDF with all the worksheets (we can tell you how to get them) and also saving the *.tax2020 data file. Then you'll at least have your original PDF and tax data file in case something gets messed up.
Folks using desktop software (CD/download) have a "What-IF" tool inside their software that allows them to experiment without changing their original return. But that feature is not available in Online TurboTax. Desktop software also can prepare multiple returns, so desktop users can quickly make another instance of their return for experimenting.
Folks using Online TurboTax might be better off to experiment using the Online TaxCaster tool: