As your notebooks grow and change over time, you might want to 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 — deletions are permanent.
Important: Unlike the OneNote desktop app, where you can use Ctrl+Z to undo actions, deleting sections in OneNote Online is immediate and permanent and cannot be undone.
In OneNote Online, go to the section you want to delete.
Right-click the section tab, and click Delete.
Acknowledge the warning that appears, and then click Yes to delete the section and all the pages within it.