How do comment voting and flagging work?
How do I upvote or downvote comments?
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You can up-vote comments by clicking the up arrow right next to them (you have to hover over the comment to see the arrow).
Down-voting of comments is not possible.
Also: this might be interesting:
You probably need a bit of rep to vote on comments too.
you do to vote on questions. https://stackoverflow.com/faq