To apply the update, click the File menu inside of Steam and then select "Check for Steam Client Updates...". The changes include:

Steam Client

• Fixed not correctly pausing any current downloads when launching a game
  
• Improved cache mounting checks. This should decrease the startup time when launching Garry's Mod
  
• Fixed startup to only request info about apps once
  
• Fixed rare cache corruption caused by defragging
  
• Fixed vgui crash when hitting pageup/pagedown in a sectioned list-panel
  
• Fixed rare crash when downloading
  
• Fixed -voicerelay steam command line to actually force use of the relay