If you simply want to override it, stick help as a command in your plugin.yml as you usually would, and add a HelpCommand class that implements CommandExecutor. Then just call getCommand("help").setExecutor(new HelpCommand()); from your onEnable method and override the onCommand method in your HelpCommand class - then you can give it any behaviour you want. Plugins are prioritised over Bukkit & Vanilla commands where they exist.