For every module, two exams are offered per semester. As long as you participated in the course for the first time and took the first exam, you have the opportunity to improve your grade in the second exam.

That also applies, if you already passed the first exam. It is not possible to worsen your grade, the better grade always counts. You need to have participated in the first exam. This is also explained on the examination board's website (in German).

In addition, there shall be an exam review for every exam. It is definitely worth going there. It is always possible that the lecturers have overlooked points or that you should have received more points at somewhere.