Walk back through the vehicle expense section. There is a question that asks if you want to claim the actual expenses or standard mileage. Here you can choose to change it back to actual expenses.
If you can not get the question asking if you want to use the standard mileage or actual expenses, then you will need to delete the vehicle and re-enter it. Then when you get to the page asking which deduction method you want to use, you can select I'll stick with my actual expenses.