Go to transport details page,as shown in the Fig 1

Fig 1
Select the vehicle number, you can see all the users for that vehicle. Click on edit button which will be displayed to the right side of each user against the user for whom the destination needs to be changed as shown in the Fig 2

Fig 2
Select the new destination point from the drop down and click on save as in Fig 3
Fig 3
The destination point of that user will be changed.