As long as you have not yet FULLY filed, it will allow you to make any changes needed to your Return.  From your Turbo Tax Timeline, click in the section where you listed your vehicle Information, it will have an edit button to change the vehicle all together or an add button to add in the vehicle in question for that month.  Click to make the adjustment, from that point it will allow you to move forward to complete your filing.   Hope this helps!...