In order to change your field of study, you must:
1. Inform yourself about the possible fields of study
2. Register for the next semester as usual
3. Fill out the Application to Change Field of Study and submit it along with your certificates (high school diploma, certificate of a university degree, certificate of assessment exam from Studienkolleg, for applicants from China, Mongolia and Vietnam: certificate from the Academic Assessment Office of the German Embassy) to the International Office. Certified copies are not required for an application to change your field of study.
Important for non-EU citizens: You have to register your change of subject with the relevant immigration authority!