How can I go back into the calc of Line 21 Form 4797? I must have entered something incorrectly and I can't override Line 21. I need adjust the cost of the property.

Depending upon how you input it, it'll show up as Sale of Business Property.  To get there:

1.  Click Federal Taxes across the top.

2.  Click Wages & Income across the top.

3.  Click I’ll choose what I work on.

4.  Scroll down to Business Items.

5.  Click Start or Update by Sale of Business Property.

You can look at/change your cost on the screen below.

I went back to review that and it didn't help to revise the Line 21, the cost of the property.  Please advise.

you can change the data that shows on line 21 by changing the number in the above field.