using CMS.Polls;

...

       string pollName = "testingSitePoll"; // Poll code name

        // Get poll info for the specified code name

        PollInfo pollObj = PollInfoProvider.GetPollInfo(pollName);

        if (pollObj != null)

        {

            // Get all answers of the poll

            DataSet ds = PollAnswerInfoProvider.GetAnswers(pollObj.PollID);

            if (!DataHelper.DataSourceIsEmpty(ds))

            {

                // Get first pollanswerinfo object

                PollAnswerInfo pollAnswerObj = new PollAnswerInfo(ds.Tables[0].Rows[0]);

                if (pollAnswerObj != null)

                {

                    // Increment votes by 1

                    pollAnswerObj.AnswerCount++;

                    // Save the data

                    PollAnswerInfoProvider.SetPollAnswerInfo(pollAnswerObj);

                }

             }

          }