I want the sign up script and the one that verifies email addresses.
A vague question is going to get a vague answer.
I for one don't have a clue what you want to actually change so I could say well it is "scripts/newuser.inc.php"
There are to many scripts and settings to deal with, to give a flat this is the file to edit.
Unless you provide a question with at least one example of what you specifiably want to change to what.
It could be a simple style change to alter the appearance of the email.(Style editor)
Or change the content of what the email sends.(mostly language file editor)
Or change the registration page.(could be a template file)
Or a control panel setting changed.
Or a code modification to completely change the way it all works.
Or a combination of all the above.
I am sorry, but I don't think anyone can properly answer the question as posted. And be of any use to you.
