"Upvote" using the like button (looks like a heart). There's no downvoting, because that's largely considered a wholly nonconstructive form of feedback. Downvotes are a lose/lose. The person being downvoted doesn't know why and, therefore, probably won't learn. The downvoter doesn't learn how to properly articulate why they thought the comment was bad. Have a problem with someone's comment? Use your words.