How do I change the graphics settings after the initial launch of the game?
Go to the directory where you installed the game and enter the Binaries\Win32 directory, run the executable bmlauncher.exe.
The default location is:
C:\Program Files (x86)\CapsuleGames\Batham Arkham City - PC\Batman - Arkham City\Binaries\Win32
on 64bit windows, I suppose on 32 bit it would be
C:\Program Files\CapsuleGames\Batham Arkham City - PC\Batman - Arkham City\Binaries\Win32
Source
Steam gives you a way to launch the configuration program. Just right-click on Batman: Arkham City™ in your Steam Library and choose the "Configure PC Options" menu item that appears.