That's a question many people have thought about. It turns out that if there's no parallel universes, the Novikov self-consistency principle should hold and that there is no way to change the past. What has happened cannot be undone. Whatever actions from the future are already accounted for in what has happened.
So, the movie The Terminator agrees with this principle: The terminator going back in time created a consistent loop that would create John Connor who would then get into a situation where a terminator must be sent back in time, and in the process would create John Connor, and so on. However the Terminator 2, 3, and 4 movies deal with changing the past/history, so they are less realistic according to this principle.