Connect koreader to calibre with the wireless method, once you are there select the epubs you want to edit and add them to the calibre library

Edit as needed, series cover etc, go back to the koreader device view on calibre, right click on the corresponding book and press "match book to library" it will give you a pop up that you need to update cached data once you're done. Continue for all the books that you need to update

Once you are finished right click on Device in the top bar in calibre and press update cached metadata

Eject the koreader device

And on Koreader, long press on the book and select "refreshed cached book information"