Well it could be anywhere in your public_html folder. But it'll probably be in the same folder as your index.php file. You can edit it using a your normal editor or use cPanel's file manager.
NOTE:
If you are using cPanel's File manager you will have to tick the "Show hidden files" box.
When you click on the "File Manager" button in cPanel it should bring up a box in the middle of the screen, in that box there should be the option to "Show hidden files".
You could also use FTP, as that'll show hidden files by default.