Time travel is fictional. There's no actual rules to how it works, so any story you see it in will have different rules depending on what that writer believes would make for a more interesting story.

In that film, they're not using the idea that you can go back in time and change the future. Instead, they're saying that when you travel back in time you create an alternate timeline, a different future where events played out differently because of your actions in the past.

So in that story, it doesn't matter if you go back in time, you can't change the future you came from. You've just created a different future where a different version of you will live.