By default, OneNote Automatically syncs your Notebooks however, The Automatic Updates are sometimes slow or do not work. In which case you should try to Manually sync the Notebook.
To Manually Sync a Notebook In OneNote
Right Click on the Notebook you want to sync
Select either the Sync This Notebook Now or Notebook Sync Status option
From the Notebook Sync Status option select either Sync All or the individual Sync Now buttons to Sync the Notebooks
