I think that we have more power than most people realize. However, when the idea of 'changing history' is brought up, people are quick to think about mysteriously going back to some time in the past and affecting the situation to change the results that we see in the present. The problem (as several people have inferred) is that there are immense factors that contribute to any specific event. So imagining traveling to another time, and as one single person 'changing history' seems out of reach.
I think we do really have an immense power to ‘change history’, but perhaps not as most people might think through the use of some magical external machine. Our real tangible power derives from the ability each one of us has to change the present through our actions. It is in the power of our actions today that history is changed. Consider what we mean by the concept of 'history'.
History is not merely a collection of facts. Facts make sense only within the milieu within which they are understood. Without a milieu… without a context, facts are meaningless. So to ‘change history’ one must act to change the current environment so that it reflects a new meaning of past events.
Marilyn can talk to the younger generation, through our expressions of and about her. Heath and all that he was has (and continues to) influence those around and they in turn will express Heath through their works and the things they do. Hitler performed atrocious actions and we though we can't change the facts of those actions, through our awareness and our activities we can change the world to be a better place where other 'Hitlers' never gain such power (or perhaps some day don't even exist). We have the power to make the tortures and deaths (though we can't change the circumstances of the individuals involved), become a point of awareness and a catalyst for better times.
That is our power as awareness-seeking beings. That is our responsibility within the flux of past, present and future. In the present, we act through what the past has presented to us... to change the future. Every action that we take affects what will come to be. And what will come to be will be the milieu within which people will look back at past events. That is how we ‘change history’. If you are interested you might look to
http://www.Changing-History.com for further discussion.