You can change the list of sections a student is attending by going to the STUDENTS section in management mode, clicking the three vertical dots on their line and selecting "Edit sections". You get a dialog with two lists: selected sections on the left, and other available sections on the right.

To add a section, you can search for it on the right side, by its title, code or teacher, then press the "+" button on its line.

To remove a section, on the left side, simply press the delete button on its line.

If you want to add all sections from one student for a new student in the same class, you can click "Copy sections from another student" at the bottom-left. This allows you to search for another student and copy their sections using the "Copy sections" button on their line.

When viewing the list of sections from SECTIONS, you can also edit their enrolled students.