They never got very philosophical about it, but the show appears to follow the Novikov Self-Consistency Principle: Nothing changes because anything the time travelers do was always part of history to begin with.
Following are some examples, but they are spoilers:
They go back to the battle of Jericho, and give Joshua the idea to send spies into the city, which they knew about from the Bible. They are the spies. Their interaction in the past actually causes history to happen as "originally intended". (Of course, this creates an ontological paradox, because no one actually originated the idea of sending in spies, but that's a discussion for a different time. Which better happen before 2/20!)
They go back to Pearl Harbor, and their actions end up saving young Tony's life, and result in his father's death, just as the original history required. No paradox in this case, but once again, history unfolds "correctly" because of their actions. What they did was always part of history.
Very interesting idea. If they hadn't been there it wouldn't have happened?
But they were always going to be there because they were always a part the event