You have to register for the exams yourselves in the POS.
The registration time for the exams is approximately from the 10th week till the end of 14th week after the start of lectures for a semester (for exact dates: here). Failing to register in this time frame results in a candidate not appearing for the exam.
Deregistration for an Exam can be done till the day before the exam. That means one has to deregister before 23:59 o'clock on the day before the exam to be successfully deregistered for an exam.