using CMS.Polls;

...

        string pollName = "testingSitePoll";

        // Get poll info

        PollInfo pollObj = PollInfoProvider.GetPollInfo(pollName);

        if (pollObj != null)

        {

            // Set the field values

            pollObj.PollResponseMessage = "Thank you for your time.";

            pollObj.PollAllowMultipleAnswers = true;

        }

        // Save the object

        PollInfoProvider.SetPollInfo(pollObj);