You can go back to the Schedule C and edit your business. Then find the Vehicle Expense section to edit the mileage information that was entered. You will need to go through several screens of questions again in the vehicle section before you will see the mileage numbers that were entered previously.
The following TurboTax help article will give you some guidance to get back to the Schedule C:
Where do I enter Schedule C?
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"