To change the assigned truck, please make sure the truck information is in the system by going to the Fleet management by searching Unit number. if there is no truck, please add it by this guide:

Also, please make sure, the truck is unassigned. This can be done by checking driver assigned truck column search in HR driver column

If the truck is assigned, go to the profile of the driver and delete it from there shown below and submit it:

Now, when truck is unassigned, you can assign the truck to the other driver by going to driver profile and searching the truck and submitting it in the profile view.