You can only make changes to the status of your exam order in certain circumstances. It depends on the status of your order. You can find this status in MyLogin on the exam session overview. 

As soon as your order has been finalized and the exam package has been sent, it is unfortunately not possible to make any changes. If the status of the exam session is still on 'open', you are permitted to make changes. 

Please note that it is only possible to order a completely new exam session if the changes are significant or you order a second session with additional exams.