When you first create a driver, you are able to either assign them to an existing vehicle, or create a new one.
The article will outline how to change this vehicle at a later date.
Under the ‘Manage’ then ‘Vehicle’ tabs you are able to add a vehicle.
Once you have done this, you should use the 'Edit Vehicle' button on individual drivers to change which vehicle is assigned to them.
Open the 'Assign a vehicle' form, delete the previous vehicle and then you can search to find the new one.