On the bottom of any comment (which could be a new topic or a reply), there are icons for thumbs up and thumbs down. Click the one that expresses your sentiment about what has been said.
You will see the count of all such sentiments get updated.
If you click thumbs up after previously clicking thumbs down, or, click thumbs down after clicking thumbs up, you are changing your sentiment. The total like and dislike counts will both change to reflect your choice. If you click thumbs up after previously clicking thumbs up, or, click thumbs down after clicking thumbs down, you are neutralizing your sentiment to no sentiment. The total like or dislike count will go up or down accordingly. You can change your mind at any time, but you can only have one like or dislike at any time.