If you could change something in canon of one of your current fandoms, what would it be?
In the show canon of Game of Thrones, I would change that entire Jaime/Cersei scene that happened recently to happen correctly as presented in the books. I mean there are quite several things I wished the show would keep from the books, from particular scenes to characterizations, but this is the most recent thing that many people were (rightfully) outraged about, and I feel like it was the most unnecessary thing to do even with the timeline change. It made no sense, so I would change it so it would make sense, a.k.a. the book version.
In Pretty Little Liars, I would change everything they tried to do with Ezra Fitz after revealing who and what he was, basically trying to stop the show from making him seem like a redeemable person when he is just a creepy child predator asshole. I liked the initial road they were taking him, it was perfect to how the themes of the show were, and then suddenly they are changing their tune because apparently due to Aria/Ezra being a big pairing amongst fans they are backpedaling and hoping that they make Aria forgive him despite everything he did, especially his "sacrifice" in the season finale. That was my biggest tiff with from last season, because I got really excited about it until they started backpedaling. So if I were to change something in canon, it would be not trying to make him redeemable, but make him unapologetic about his actions, and for Aria to finally understand who he is and to want nothing to do with him ever again, no matter how much he claims that he still loves her and justifies his actions. I would make him become, if not associated with anything "A" related, then just another antagonist within the show.
I would change the entire fourth season of Lost Girl. See my previous answer for more details on precisely how I would do that.
And lastly, um, I would also change the ending for The Hobbit, for obvious reasons. Nobody dies, Thorin redeems himself and is cured from the gold sickness and he either becomes King Under the Mountain and Bilbo lives with him or Fili becomes the next King while Thorin goes to live with Bilbo in the Shire, and everyone lives happily ever after, the end. (◕‿◕✿)