Here are the lines concerning changing the past and timelines
Hulk: "Think about it: if you travel to the past, that past becomes your future, and your former present becomes the past, which can't now be changed by your new future!"
The Ancient One: "The Infinity Stones create what you experience is the flow of time. Remove one of the stones and that flow splits. Now, this may benefit your reality, but my new one, not so much. In this new branched reality, without out chief weapon against the forces of darkness, our world would be overrun. Millions will suffer. Tell me, Doctor. Can your science prevent all that?"
Banner: "No, but we can erase it. Because once we're done with the stones, we can return each one to its own timeline at the moment it was taken. So, chronologically, in that reality, it never left."
