Hi Martien, currently you can’t do this except by creating a new notebook, copying and pasting all the cells from the old notebook into it, and then deleting the old notebook.
Given your post in the other thread, I’m guessing you’re well aware of the fact that duplicating and deleting isn’t exactly the same as “unforking”: if the notebook has been published, there’s no way to create a new notebook that has the same URI (for good reasons!). So it’s possible this isn’t the answer you’re looking for.
If on the other hand, this is good enough, you can save yourself some time if you’re willing to run a node.js program from the command line: see the script I posted (based on work of @mootari) in the “backup method?” thread, which has notebook “download” and “upload” functionality (at least until the API changes).