How do you go about voting on Yahoo answers? I don't know how to do that.?

Yahoo! Answers used to have a community voting section for questions that did not have best answer. If the asker did not choose a best answer it would go to a vote and the community could vote for the best answer.

That option has since been discontinued.

There is no longer any voting - only the person who asked the question now gets to say which is the best answer.

You cannot do so.