Whatever you do, do not start messing around with your filed tax return. Leave it alone or you will not be able to transfer it to your 2020 return when the time comes to prepare a 2020 return next year. If you want to prepare a "dummy" return for 2020 create a different account and put in your anticipated 2020 data--the tax laws will be pretty much the same except for some adjustments in standard deduction etc. Or use the taxcaster tool:
I have the 2019 TurboTax Deluxe on my laptop. If you are using the same product, you can still open the 2019 TT without disturbing your actual filed 2019 return. To estimate how much federal tax you need to withhold in 2020, open the federal tax tax in your 2019 TT; click on "other tax situations" at the top of the screen; when the next screen opens, click on "Form w-4 and estimated taxes" which is located at the bottom of the screen.
@dolores_Marg The person who posted the original question posted from online TurboTax. That user cannot do what you suggested--that can only be done using the desktop software. If an online software user changes anything in their return it will be a mess.