First, if you edit an existing cpanel theme, the next update will overwrite your changes. You have 2 options. Either make the changes and redo them after updates, or make your own theme. With your own theme you do need to keep it up to date with cpanel changes.
If you choose your own theme method, in WHM use &Clone a Cpanel Theme& to make your own copy of whichever one you want a copy of with your own theme name.
With their theme, or your own, the files you want to change are at
/usr/local/cpanel/base/frontend/[i:c47dd3b3d7]themename[/i:c47dd3b3d7]
Just use a secure FTP program to download the file you want to edit, or the whole themename directory, then edit and upload the changes. Just don't forget that if you edit an existing theme, you may need to redo it with cpanel updates. Not every file is overwritten so you won't need to redo it every time.