Hello players. It looks like Steam is currently experiencing issues with downloading or updating the game via US (and potentially) other download regions.
Fortunately, there is a workaround until Steam fixes the problem -- you can change your download region to one that works.
Currently, we know that the Australian regions are working, so switch over to one of those and try downloading again, remembering to clear your download cache, verify local files and restart Steam (if necessary).
For instructions on changing regions, please see the following support page: https://support.steampowered.com/kb_article.php?ref=2274-IFLV-5334
Update: A number of other recently-updated games are also affected by this issue, confirming our suspicions that it's a problem with Steam and its distribution network.