I eventually figured out your explanation, as I had the same query as "Fanysoft". I made the error of Right clicking on the notebook name that was in the sidebar. When I clicked view>notebook and then right clicked on the notebook I wanted to rename IN THE CENTRAL PANE (not the sidepane) I could see the rename option.
However the right click on the notebook in the side pane is more intuitive and if the product is to be user friendly, then it odd why in the current version (6.13.1) does not allow for this???
I looked at Microsoft's OneNote again yesterday. looks quite sw