Go to student central and ask them for the necessary paperwork.
You’ll need to withdraw from each of your courses individually, which means doing paper work for each one and having them signed off by the head of the course and lodging them with the faculty.
It’s a fair bit of effort, but make sure you follow it all through. I ‘withdrew’ in 2007 due to ill health, got fed up with the paperwork process and gave up. Now that I am re-enrolled in 2009, I am on ‘probation’ – because just not showing up to courses will give you a nice big fail on your transcript which looks like crap and make things difficult if you want to re-enroll.