On the top right of the menu bar in TT, there is a FORMS button. Click it, then choose the form you need to edit.
Why do you need to over-ride that amount?
Amount from schedule 3 Additional Credits and Payments line 15 carry over to Form 1040 line 31. That amount is the total of Other Payments and Refundable credits.
my 1040 worksheet (line 14) is displaying the correct number from Schedule 3, line 15 so the worksheet line 15 has the correct number. HOWEVER, The main 1040 form line 31 number does not change BUT the software IS DOING THE MATH. I can change the number on schedule 3, LINE 15 and watch the amount owed change on the 1040. how do I get the main 1040 to display the number on line 31
If the lines farther down the return are all correct, and you noted that you can see that a change to Schedule 3 Line 15 changes the balance due, then the program is calculating correctly and there may just be an issue with your display in Forms.
Overriding a calculation field in Forms is not recommended, as it may invalidate the terms of the 100% Accurate Calculation Guarantee.
Try exiting TurboTax, and launching the program again, allowing it to run any updates. If the problem persists, see this help article and this one for instructions on how to contact TurboTax customer support depending on which product you are using.