I entered a date I will pay underpayment penalty but I have not filed yet. Now I want to change that date before I file. How can I change that date I entered?

Step through the FILE section again and you'll come to the page asking how you want to pay your tax due.  You can change the payment method and date.

If you're using TurboTax Online, close the program, and clear your Cache and Cookies first.

@Johnny-Row