To apply a refund to next year's estimated taxes:

1. From within your TaxAct return (Online or Desktop), click Federal. On smaller devices, click the menu icon in the upper left-hand corner, then select Federal
2. Click Miscellaneous Topics in the middle of the screen to expand the category, then click Apply overpayment towards 2019 tax liability
3. If your return currently reflects a Federal refund amount, this screen will allow you to apply the refund to next year's tax liability

Note that if you need to apply a State refund to next year's estimated taxes you would do that by going to the State Q&A section of the interview (the instructions below are general as some state menus will vary):

1. Click State. On smaller devices, click the menu icon in the upper left-hand corner, then select State
2. Click the name of the state
3. Click 2019 Estimated Tax Payments
4. Click Add to create a new copy of the form or Review to review a form already created (for some states you will have the option here to click Apply Refund and be taken directly to the screen to enter the amount)
5. If your return currently reflects a State refund amount, you can click Continue to advance to the screen where you can select the option and enter the amount of the current year's State refund that you wish to apply to the following year's return

Once either of the above instructions are completed you will see that the Federal and/or State refund amount in the top right corner of the screen has been reduced or is now zero, depending on the amount you choose to apply to next year's return.