Steam should automatically download updates to the game client as they are released.
Triggering an update
On occasion, a game update may be available but the Steam client delays in downloading it.
The following instructions should trigger the Steam client to download the update:
1. Launch the Steam client software and log into your Steam account.
2. Right-click the game in the library and select Properties.
3. Click the LOCAL FILES tab of the properties window and then the option VIEW INTEGRITY OF DEMO FILES...
4. Once the verification completes the update should begin downloading.