OneNote for Windows 10 automatically syncs all of your notes for you. If you prefer, you can choose to sync notebooks manually whenever you want.
Do the following:
Click the Navigation button near the upper left of the OneNote app window.
In the navigation pane that opens, click the Notebooks dropdown arrow next to the name of the current notebook.
In the list of notebooks that appears, right-click the name of the notebook you want to sync, point to Sync, and then choose either Sync This Notebook, or Sync All Notebooks.