If I download this game again, will it automatically update the one I already have? Or do I need to redownload it and start over again?
You would have to download again but the saves will still be there. When the game updates some saves from older versions might not work. The game does warn you before the Main menu that the saves from older updates won't work. You can play for weeks to months and still be fine. Put when you need to redo, you most likely have done a lot and maybe meeting dead ends.