I don't think you can restart one story, but you can reset the whole game without uninstalling. It will tell you on the first page of the manual thingy.
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you have to uninstall the game and then reinstall it
I'd just re-install the game.
You have to uninstall it and then Reinstall it. It really stinks but that's what i did once to restart something that I messed up. If anything, Try this and this is the only thing that I know how to do it. Sorry!! Hope this helped!!
UnlockStoryRewards ON
i do not have the game, but if you go to options or settings and your save file, just delete the saved file and you can restart all over