Go to "Business" tab and "Income and Expenses" should come up. Click "Choose what I work on." Under "Business Income and Expenses," choose to UPDATE. By business name, choose EDIT. Under "Business Expenses" there is a "Business Vehicle Expenses" line that you can UPDATE. Choose EDIT from the page it takes you. Go through these questions until it asks if you have documented your mileage. Make sure both of these are selected and CONTINUE. You will now be at the page you can make the change on.
