To plan for next year you can go though the Estimate section or in the Desktop program there is also a What-If worksheet. And in Desktop, you can save your return with another name to test in. Go up to FILE – SAVE AS. In the Online version click on Add A State to let you back into your return, Be very careful not to change any of your actual current return.
You can just type W4 in the search box at the top of your return, click on Find. Then Click on Jump To and it will take you to the estimated tax payments section. Say no to changing your W-4 and the next screen will start the estimated taxes section.
OR go to
Federal Taxes or Personal (Home&Business version)
Other Tax Situations
Other Tax Forms
Form W-4 and Estimated Taxes - Click the Start or Update button
Say No to W4. When you get to the W4 and Estimated Taxes section, say you want to adjust your income to go though all the screens.
If you have the Desktop program you can do a What-If worksheet.
Go to Forms Mode, click Forms in the upper right or on the left for Mac. Then click Open Forms box in the top of the column on the left. Open the US listing of forms and towards the bottom find the What-if worksheet. It's right under Estimated Taxes.