|I don't use cpanel persay (more of a clone), but after you log into cpanel, you should see a layout for various things related to your site account - look for something like "file management" and follow the link to anything resembling an online file manager.|
From there, navigate to your site's root folder to find the default page being served - something like index.html or index.php. There's usually a few icons for editing, viewing, creating, deleting, etc files/directories and checkboxes for selecting any file(s) you wish to work with.
If you ftp access to your site, you might find it easier to use an ftp client... modify your scripts locally and then upload them to the appropriate directory. Note: you may need to set file permissions using this approach.