Let's consider Einstein's marble universe. There's no changing it. Change is the dimension called time. Yes, you can go back in time; it won't change the future you came from.
But Einstein failed to deal with Schroedinger's Cat. That means that that time is not one dimension but at least two. The observer cannot determine the outcome of the experiment but only which observer he is.
In short, you can go back in time, but it won't change anything. Evry present has multiple pasts and multiple futures. Causality is not what you think it is.