how can u help a pessimist change his/her outlook?

Is it absolutely necessary? Actually to be very clear, I do not think anyone can change the ideology that someone else possesses. Changes come from within. Have you seen the Hindi movie 'Ashok'. How did the change come? It is possible only through realizations by the self. However,you may create instances, that may or may not work. Do not worry. These are all in the hands of god. After killing so many people, Ashok realized that his path was wrong. Whereas just seeing one arrow-hit bird, Siddhartha's entire thought process took a turn. You have to wait for the right time, the pessimist will change on his own...

Only by example can we "change" another person's outlook. If a pessimistic person continually sees benefits from another point of view, they may seek to find out why.