I think you mean the "Browser" in the main menu. Open ~/.config/openbox/menu.xml in a text editor. Scroll about halfway down and you'll see a section with <menu id = "root-menu" label="Openbox 3">. Look for the line <item label="Web Browser">. The next line down, beginning with <action name=, you'll see the name of the web browser being called. Change just the name of the executable that calls the browser you want. For example, to call Firefox, the line would read:
<action name="Execute"><command>firefox</command></action>
(~ is your home directory)