You can cancel course registrations at uva.nl/courseregistration. Go to the ‘Overview of registrations’ tab, check the box next to the course you wish to cancel and click on 'Deregister' and then on ‘OK’. You can only cancel courses up until the end of the first week [or an alternative restriction] after the start of the course.
Please note: it is important to deregister for courses you do not plan to follow. If you do not attend a course for which you registered, the result recorded for this course will be ‘NA’ (the Dutch abbreviation for ‘non-attendance, counted as exam attempt’). Moreover, you will be preventing a fellow student from occupying your place in the course.