This depends a lot on the country and university specifics. However during my studies once happened for me to observe students replacing professor they deemed it is not competent enough for the particular course.
The student group should apply to university management asking to replace the teacher as not good enough. Of course, such application must list multiple factual errors in presented material, uncovered topics that you consider important for the subject, examples of undelivered information that you think would be highly relevant to the given lecture and the like. Significant number of students should apply so it would be difficult just to represent this as a personal conflict.
If the teaching person is not a head of laboratory / department but instead is under supervision of another competent professor, you may also talk to his supervisor. Some universities concentrate on research so deeply that teaching may be delegated to somebody more junior and less experienced.
If you cannot find enough arguments or supporters for this procedure, the professor is actually competent ...