How to modify the header  in superfish menu?
I'm not sure that this is a good idea because changing that ID may break the scripted functionality of a Superfish menu but that said, here goes...


I'm assuming you're on Drupal 7.


It's not a template that you'll be doing anything with. It is a theme function. When the Superfish menu is generated by drupal it is in response to a call to the theme_superfish() function. You can find detailed info info on it here: http://api.drupalhelp.net/api/superfish/superfish.module/function/theme_superfish/7

So with that in mind, you would want to override that funtion in your template.php file in your theme folder by copying the function to your template.php file, renaming it to YOUR_THEME_theme_superfish($variables) and changing the part at the end where it builds the menu. 

If you have several Superfish menus and only want to affect one of them (ie: superfish-1), you can add a conditional to the function so that it only changes the ID of the menu when the conditions match.

Hope that works for you.