Kobo do have a user guide - I saved a copy of the PDF here (back in 2014, so not certain it is still current) but don't have a copy of the link (though I seem to remember it being hard to find).
Under the heading of "Editing your content" it says
Quote:
After your eBook has been successfully published to the Kobo eBookstore, you
can edit its information at any time. All published eBooks can be found in the
“eBooks” tab, under the On sale category.
Select the eBook you wish to edit in the eBooks tab. From the dropdown menu
that appears, you can choose to edit the following:
[...]
Change eBook content
Selecting this link will allow you to edit the content inside your eBook.
[...]
We can apply your changes within 12 – 24 hours. For our partner retailer
stores, it generally takes an additional 24 - 48 hours before your changes come into effect.
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I do publish on Kobo but I've never used that feature so can't help you more. As Dr. Drib suggested, an email to the site may give you a more detailed response.