The page you are looking at is being generated dynamically by CodeIgniter.
If you would like to edit this page you'll find it located at:
application/views/welcome_message.php
The corresponding controller for this page is found at:
application/controllers/Welcome.php
If you are exploring CodeIgniter for the very first time, you should start by reading the User Guide.
If you using RESTful API, you should reading the API document.
If you are an developer, maybe you need update API document editor.