Drivers aren't tethered to vehicles in AGG SMART, they can move freely between them. The drivers are not assigned to a vehicle, rather they log in to a vehicle when they enter the app. If you ask your drivers to go to the Driver tab on the bottom of their AGG NAV app, and then choose Log Out, this will remove them from their current vehicle.

They can then log back in with their pin number, and choose a different vehicle to log back in to from the drop down list provided. If the vehicle required is not in the list, this is because said vehicle is already in use on another driver's app. If you need to log this driver out, but they are not available to follow these instructions, see this article.