If you need to withdraw from a single course (as long as it is not your only class) in the Fall or Spring semester, for most classes, this is done in SIS; see the University Registrar's page on class ...

Students may repeat any course in an effort to improve grades and/or to improve understanding of the course content. Students are encouraged to seek advice of an academic advisor before repeating ...

Non-matriculated students are considered part-time and are permitted to schedule less than 12 undergraduate credits per quarter (fall, winter, spring and summer). This option allows you to select ...