[eluser]xwero[/eluser]
If you want to edit it you better extend the helper. the helper name is form so you create a file application/libraries/MY_form_helper.php and you add the function form_fieldset where you add your code.
If you load the form helper your customized function will be available.
[eluser]Derek Allard[/eluser]
If all you're wanting to do is add css classes to these, both anchor and form_fieldset already support this. There are examples in the docs, and for fieldset that exact example (setting a css class) is used.