You should find answers to questions regarding examination law in the examination regulations (PO) and module handbook of your study program. Read through these carefully.
In particular, you must register for the exam in the BOSS system two weeks before an exam. To register for an oral examination, you will need a form (German only), which you must complete and send to the central examination administration by the due date.
For written exams, you can deregister from the exam up to one day before. For oral exams, you can only deregister up to one week before the exam. Please note that you must deregister both with your examiner and in the BOSS system.