It's simple! If you want to change the answer to the poll from your phone (whether you're the creator or have downloaded an app with a poll on your phone), just click 'Return to Poll' from the results page.
Then click the answer you'd like to change to and tap 'Submit' again.
If you are the creator of the app, you can also change your answer in the editor online at http://yapp.us by following the same steps.