To override a field, you switch to Forms, find the specific entry you want to override, and then right-click or double-click to bring up a menu that includes "Override". Choose that, then delete what's in there automatically and type what you want.
The best thing is usually to override the first place a mistaken result appears. For my case (refi), that's the "Average balance" line in the "Loan 1" column in Part 1 of the "Ded Home Mort" worksheet (which panel will depend when your loan originated).
You may need to override a few other fields in Part 2, since it appears TT is adding up the interest from the two loans incorrectly (at least in my case).
