How do I get to the Federal Tax withholding area in TurboTax?

Taxes withheld would be your W-2 or Form 1099.  Go to those sections and edit the form you entered.

Click on Federal>Wages & Income

1. Click on Federal Taxes
2. Then click Wages and Income
3. Click Continue and I'll choose what I work on
4. Click on Update to the right of Wages and Salaries, Form W-2
5. Click on the appropriate W-2 and update the withholding