Hi leela - depending what you're doing, you may need to look in different files. First, I would check login.html (<Install Directory>\9.x\vizportalclient\public\<language>). Note: if you have users viewing the page in multiple languages, you may need to adjust the login page in each folder.
Be warned, though. Making these kinds of changes is officially venturing into "unsupported" territory, and if you contact support with an issue that they believe could be caused by one of these "customizations", they will generally insist you revert to an unadulterated version of Tableau Server before they will continue to look into it further.
Thanks Ben Neville.