You can swap classes in the 'My Timetable' tab of the Student System. In the 'My Timetable' grid underneath your topic listing, you can see if a class is swappable by the swap icon.
To swap, click on the 'Change' button next to the relevant topic listed in My Timetable and then you will see the available options listed in the My timetable grid at the bottom of the page. Click on the plus icon to select the class.
Click on the 'Save' button to save changes.