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 can make changes.
If your order has been finalized already, you can cancel the exam session order and place a new order.