Yes you can. In the question's page, you have “Thumbs up” and “Thumbs down” icons in front of the question as well as all its answers.

You vote up a question/answer by clicking the Thumbs up button. As a result, the number of votes corresponding to the question will increase by 1.

Likewise, you can vote down by clicking Thumbs down and therefore decrease the number of votes by 1.

Once you vote, the button's color changes, allowing you to see easily your votes history on that page.

You can only vote a question/answer once. You cannot vote your own question or answer.