Where do I enter what potion of my investment income was withheld for local/state taxes (Form 8960, line 9b) in TurboTax software?

You would need the desktop version of TurboTax to change that figure. You can enter it directly on line 9(b)(2) of the Form 8960 worksheet. You would access the form by using the Forms menu option, where you will see a list of the forms in your TurboTax program.