I need to pay some Federal underpayment penalty for 2018.

In the 'Actual Withholding' section, I tried to use 'elect to treat the tax withheld as paid when it was actually withheld' option.

Then I changed my mind and realized that I should use the default 'four equal quarterly installments', but even I selected 'No' for 'elect to treat the tax withheld as paid when it was actually withheld', Turbox tax still uses my original input for 'actual withheld' but never goes back to equal quarterly installments.

How should I let Turbox abandon my inputs and use the default values?