Calibre is a free ebook management software that has conversion and editing capabilities. Download here. Next steps:
Add your Word document to Calibre as a book.
Click on “convert book” and select EPUB format.
You can then edit the EPUB in Calibre (which I don’t find very pleasant—but it’s possible), or you can open it up in Sigil for further editing. To risk repeating myself a lot, this specific process will end up creating more cruft (garbage code) than you probably want or need, but it gets you an editable EPUB file.