Here's a few things you can try:
-Try verifying your game files.
-In the client, in the Steam menu select Settings, then Downloads, and try changing your Download Region.
-Try logging out of Steam, exit the client, reboot, start Steam again, log in, and see if that helps.
-Try 'clearing the download cache' and reboot your computer (in Steam, Settings, Downloads section).
https://support.steampowered.com/kb_article.php?ref=3134-TIAL-4638-Try checking your drive for errors. In Windows Explorer, right click the drive and select Properties, switch to the Tools tab and click Check Now in the Error-checking section.
-Make sure you have enough free drive space to install the update (twice the download size).
-Try exiting out of the Steam client, then browse to the SteamApps folder where the game is being installed and delete the contents of the 'downloading' folder. Right click on the Steam shortcut and run it as administrator and try installing the update again.
