# dynamically: list tags, edit one with sed, rewrite tags
vorbiscomment -l file.ogg | sed 's/^genre=.*/GENRE=Black Metal/' | vorbiscomment -w file.ogg

# with a temporary file (it's actually one of the examples in the manpage)
# (a bit longer/more difficult to use in a for loop,
# since you can specify only one input file)
vorbiscomment -l file.ogg > comments.txt
<edit comments.txt with vim/nano/sed>
vorbiscomment -w -c comments.txt file.ogg