How can I override next year's TurboTax's estimated tax payments on form 1040-ES

You can either just cross out the number and write in the amount you want to pay or you can go through the estimated taxes section and change the input. Type estimated tax payments for next year in the search box and click on jump to. Skip the change W-4 screen. None of the estimated tax amounts get sent to the IRS until you include the form with your payment.

Crossing out the number is just a hack . . . 

TurboTax in previous versions opened up a section on estimated taxes rather than just doing the calculations in the background.   The process is under the Federal Tax - Other Tax Situations.   I just found that this is a gianormous PITA when all I want to do is revise the estimated tax payment.  If you actually go through the ENTIRE process again, you STILL cannot edit the stupid amount.

I just realized why you do this hack (not that we should need to do this)

I was able to do this in TTax 2020.  

Go to forms: Scroll down to Est Tax Options, and select the form

Any field you want to change, right-click, and select Override.  Put in the amount you want to use.