Start a New Game+
Complete Chapter 1 of the game, Hometown.
As soon as Chapter 2 begins (20 Years Later), pause the game and save. Exit to the main menu.
Choose Chapter Select and select the first chapter "Hometown"
Notice you can now select Survivor difficulty? Select it and play the game from this point on to the end.
You can't start a New Game+ without finishing the game once already. Survivor shouldn't pose too much of a difficulty. The only bits I struggled on were the Bloaters which I sneaked past most of them. The only two I did have to fight were the one in the high school and the one with Ellie when you're fighting off waves of enemies with David as they are unmissable. If you're struggling at all there's a video I've attached to help, but you probably won't need it. Good luck!