So, if you're already playing this game, I'm assuming you know how to find the game's files. If you don't here's a quick refresher:

1. Go to the game's icon in your steam library. See that bar all the way to your left with the list of games you have? It's there.
2. right click it.
3. Click "browse local files."

1. In the files, enter the folder called "game."
2. Scroll until you find a file labled "firstrun."
3. Delete it.

All you need to do now is boot up the game. When it asks you if you want to delete your date, simply click "yes." And voila, you've started the game over again!