GUYS GET THIS TO THE TOP! CLAIM YOUR STEAM KEY THAT YOU GOT WITH PURCHASE, DOWNLOAD STEAM TO YOUR COMPUTER AND FOLLOW THE YOUTUBE VIDEO TO CHANGE TO A PUBLIC BETA! THIS IS HOW TO UPDATE!
Or alternatively, DON'T use Steam because for some people it's a health hazard and has been known to be such for over a year, with Valve's response to that knowledge being to wait 6 months then start actively blocking access to the only safe workaround anyone had been able to provide.
You just need to re-download (which gets you the most recent copy of the game), extract the files to the same place as you had previously installed the game, and tell it "yes to all" when it asks to overwrite your files.