As your notebooks grow and change over time, you might want to delete sections that you no longer need. You can easily remove sections from your notebooks using OneNote Online, but be sure that you really want to get rid of them — deleting a section is permanent, and all of the pages within that section will also be deleted.
Important: Unlike the full-featured OneNote apps, where you can use Ctrl+Z (Windows) or ⌘+Z (Mac) to undo your last actions, deleting sections (and all of the pages they contain) in OneNote Online is immediate and cannot be undone.
In OneNote Online, go to the notebook containing the section that you want to delete.
Right-click (PC) or Control-click (Mac) the section name, and then click Delete.