The idea of altering the course of time is deeply and inherently illogical on he most fundamental of levels. Going back into the past will not change the past, only make the time traveller part of the past.
"But what if I go back in time and assassinate Hitler? Won't that alter the course of history?"
We are all familiar with the paradox of going back in time to alter the course of history: If one were able to travel back in time, and succeeded in assassinating Hitler, suddenly the future would be one that never had a Hitler in its past. But then there would not be a reason for the time traveller to have gone back in time in the first place, so Hitler wouldn't have been assassinated by the time traveller, so he would have come to power, so the time traveller would have gone back to kill him, so he wouldn't have come to power... and so on and so on ad infinitum.
The reason this paradox exists is because the problem is based on flawed assumptions. In reality, going back in time would not alter the course of history at all. The going back and appearing at some point in the past would be precisely part of the series of events leading up to the going back. Whatever the time traveller might do in the past is part of the flow of history which leads up to his travelling. The true timeline would actually be like this:
At some point in history, this time traveller guy shows up out of nowhere, walks around a bit and does stuff that affects the subsequent course of history, and eventually disappears again. History plods ever onwards, until one day this guy is born who grows up to invent time travel. He tests his time machine, disappears for a while,and then reappears at the temporal coordinates he set into the machine before disappearing. History plods ever onwards.
You see, what he did in the past is the past, by definition the series of events which leads up to the present. The past he goes back to is the very past which leads up to his present moment in which he turns on the time machine. Whatever he might do to try to kill Hitler while in the past is necessarily part of that series of events which lead up to Hitler's rise to power, and as such only contribute to Hitler's ascension, rather than prevent it.