The Exchange button next to the Drop button will appear on the Exam details screen for the exams you hold a valid registration to, and the registration period is not over yet. You can register to another future exam date of the course by appliying the Exchange button.
Switch exam date
Click the Exchange button, then in the popup window select the exam date of the course you would like to take. The registration for the new date will be completed by clicking on the Switch exam date button.
Tip
If the change of exam date is unsuccessful (e.g. the exam is full or other registration conditions are not met), your original exam registration remains valid. If there are only a few places for an exam, it is better to choose the switch of the exam date instead of the two-step deregistration-registration, so your registration for the original exam date will remain valid.