If you think about Einstein's theory of relativity, you'll realize that time is not a straight line, but is a circle. This means that the future has already been set. But the question is, if this is true, then how can we alter events when the future has already been set?
The future hasn't been "set". It's just already happened. Kinda like the past, but we're not there yet. You can't alter something that's already happened, like the past. But you don't know the future, so you can't alter it. Travel to the "future" is not possible. You can only travel to the present. Ever. According to Einstein's Theories, If you travel at close (95%+) to the speed of light, time will slow down around relatively. So if you travel for a day, and come back to earth to find that a year has passed (which is, in theory, possible), have you really traveled to the future, or just slowed down your present? As Humans, we are perpetually stuck in the present. We can slow down time around us, but we'll always be "here", "now", and we'll just have to accept that.