If this is kindle ebook then you can try this:
Calibre by itself won’t remove it. You need at least the de-drm plugin. With kindle there are the other very specific steps outlined in Peter’s post.
Ultimately epubor is probably the most straightforward. I’ve not used it and have always successfully done the Calibre route, but those who have seem to like it. Ultimately I may need to too…I think Amazon has been going to a new DRM flavor on its newly published books that I think even epubor couldn’t do initially (but I think maybe have since then figured it out??).