If you have entered a trip and it has been approved, if the date or time for your trip changes, you will have to reschedule the trip. When you reschedule your trip, if there was a vehicle and/or driver assigned to that trip, the vehicle and/or driver will be removed. The vehicle owner at your location, or an admin user at transportation, will need to go back in and assign the vehicle and/or driver again for your trip. 

To begin rescheduling you need to open up the request that you need to change the date and time on :

Scroll to the bottom and you should see the Reschedule Trip button :

Once you click to reschedule - a box will open up where you can select the new date and time for your trip - be sure that you click the OK button to save your changes :