You have to change the save file. look for a GameProgress field and set the number to 2 I think (if not that, then 3). You could skip checkpoints, but you may miss a bunch of crucial progress triggers, so I'd just play from the start if I were you
For future reference: after 2 minutes of gameplay, there was no "GameProgress" field in the save file, I assume because I hadn't finished any chapter. I added it and set it to 2, and it worked like a charm.
