Once you have TT installed on your computer and you open your tax data file, you can switch to Forms mode (icon in the top menu bar) and see a list of tax forms. There is a home mortgage interest worksheet (called Mort Int Wks or something like that) but there is different worksheet where the limitation is calculated (whose name I forget).
Final numbers like mortgage interest on line 8a of schedule A usually can't be directly edited, but double-clicking on the number will usually take you to the worksheet where the number was calculated. There is an override command in the Forms menu that will allow you to directly enter a number into a normally calculated space, that you can use to enter the correct interest amount when you are ready to amend.
