How to edit/modify Help button in GUI using Skill

I'm assuming you're talking about a form you're creating yourself? This is covered in a section in the help headed "Specifying Your Own Callback for Help" - try searching for that in cdnshelp. Here's a small example:

hiCreateAppForm(
    ?name 'MyHelpForm
    ?formTitle "How to customize help"
    ?fields list(
        hiCreateStringField(
            ?name 'entryField
            ?prompt "Enter something"
        )
    )
)
MyHelpForm.hiHelpAction="MyHelpCB()"

procedure(MyHelpCB()
    system("evince /xyz/abc/test.pdf")
)

hiDisplayForm(MyHelpForm)

Hey Andrew,
thank you very much
yes am talking about the local form that i have created.

am referring to  this "Help" option.

if i click the above help option am able to open the pdf file which exists in path  /xyz/abc/test.pdf