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 though 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.