- Launch Steam
- From the 'Library' section
- Right-click on the game
- Select 'Properties' from the menu.
- Select the 'Local Files' tab
- Click the 'Verify Integrity of Game Files'
- Steam will verify the game's files - this process may take several minutes. This will always download a number of files even if you've just verified.
NOTE - If you are using a Real Name Fix then you will have to delete the necessary files again upon the completion of this process