I've got a responce from iBookstore Publishing Support - actually I have to use iTP to accomplish required tasks:
"Thank you for contacting iBookstore Publisher Support regarding the request to update your book's screenshots and categories.
We have made a temporary change on our end to allow you to update your screenshots and categories for your book with a redelivery using iTunes Producer. Make sure to redeliver all of the screenshots you want to display on the iBookstore in a timely manner.
To update your screenshots, open the book package in the latest version of iTunes Producer by clicking on Look Up Metadata. Then make sure all required metadata fields are filled out (or added if necessary), upload all of your screenshots in the Screenshot section in the Assets tab, and click Deliver when you are ready to deliver the updated book to us."
So, this is how it's done.
Note to myself - when creating and publishing a new book, have to make sure that I don't do drastic changes to a new version, so the description, categories, and screenshots (things that's hard to change afterwards) stay the same for all editions.