I think in the Estimate section is just to apply a refund from 2018 to the current year 2019.

To apply your federal refund to next year, enter it under

Federal Taxes Tab or Personal (Home & Business)

Other Tax Situations

Additional Tax Payments

Apply Refund to Next Year - Click the Start or Update button

To apply your state refund to next year, enter it under

State Taxes tab

And keep continuing and stepping through the interview about a dozen or more screens and it will finally ask how much you want to apply.  It is probably one of the last questions in the state section.

Is it possible to undo this? I checked it by mistake and now I can’t find out how to undo it. 

Did you file it yet?  If not then go through the State tab again, probably near the end and look for that screen again to change it.  Change it to No and delete the amount.