You can find all the registration instructions here (please note that the following linked pages are in German):
The following applies to all final module examinations registered with the Examination Office:
You may withdraw from the examination in writing (and also by e-mail) to the Examination Office no later than one week before the examination date without giving reasons. If you do not meet this deadline, you must immediately inform the Examination Office of the reasons for your withdrawal or for missing the examination and send in a medical certificate.