To plan for next year in the Online version you can go though the Estimate section. Click on Add A State to let you back into your return, Be very careful not to change any of your actual current return. Or you can set up another account to test in. You don't have to pay unless you want to print it out. You can use the same email address for 5 accounts. How to start a new online return
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-onli...
It would be better to use the Desktop installed program. 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.
To use either you can go to the Estimated section. 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.
