All things in the application can be moved around with drag and drop. Simply click and hold a section in the table of contents and then you can move it up and down and drop it anywhere on the list.
If you want to make a section into a subsection, drop the section onto the section you want it to be a subsection under
Sections get renumbered automatically. If that doesn't happen try to reload the page.
See also How to build the table of contents