Notebooks can be exported via the notebook settings. To open the notebook settings click the notebook cover in the navigation panel on the left side of your screen.
Decide in the following window if you want to add the comments and/or the archived tasks (green means included). If you had activated a filter before opening the menu, you can also decide whether you want all the items or only those of the filter.
Once you have added and selected everything you want, click "Generate JSON". You can either copy the file to your clipboard or download it.