You can use onenote desktop app to back up your notes through File > options, choosing save and backup, selecting folder to save in and pressing OK.
You can find detailed steps here : https://support.office.com/en-us/article/Back-up-notes-f58b34b0-611d-435e-87fa-7942a1767af4
You can also sign in to you Microsoft account in onenote and all your notes will be synced to the cloud, if you want an easier way to keep your notes backed up.