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 (if your district allows you to reschedule on your own). If your district does not allow end users to reschedule trips you should receive a message box on who you are supposed to contact in order to reschedule. 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 (they will be notified via email). 

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 :