I'm afraid I'm doing a mistake.
Standard type OO/LO Writer files are the source

If it is already an EPUB, use Sigil, Calibre's Editor or some other EPUB editor.

You could use the Unpack EPUB tool , to work on the individual files inside the EPUB, but that implies you are not changing the file structure of the book (it is mostly simple typo fixes, replacement of pieces, not add or delete)
Ah, ok. Many thanks.