Use calibre book converter. You might need a plugin if it’s a super specific drm that needs to be removed (like, say, a library book from the libby app), but calibre will let you pick any output you want and save it as a clean file.
download it to kindle 2: transfer azw/mobi file to computer 3: open calibre and install DeDRM 4: drag and drop books into calibre 5: calibre is auto de-drm’ing the books so you can now convert them to any format you want or just backup the books