This season of Game of Thrones is remarkable for a number of reasons—the show has left the books behind, the plot is picking up pace, Someecards' new Snapchat recaps of episodes—but the most noticeable one is time travel. (Spoilers, obviously) Bran Stark and the Three-Eyed Raven take several trips into the past, and after leaving the Raven, Bran pretty much montages all of Westerosi history.
But how does it work? At first, these seemed to be "no touching" trips—flashbacks Bran could walk around in. But then, well, Hodor. Game of Thrones has a particular flavor of time travel, one that's a lot more complicated than Back to the Future. Fortunately, Vox breaks down the mechanics of this time travel, as well as its implications for the future of Thrones.
So basically, Bran has already gone back in time to affect everything in the plot...he just still needs to do it when the time comes.