What's the Relationship of the 2 header.php ?

What's the Relationship of the 2 header.php files?
There are 2 files named header.php located in the directory
/includes/languages/classic/ and /includes/languages/.
They all have the following section:

define('HEADER_ALT_TEXT', 'Powered by Zen Cart');
define('HEADER_SALES_TEXT', '<h1>Zen Cart</h1>');
define('HEADER_LOGO_WIDTH', '200px');
define('HEADER_LOGO_HEIGHT', '70px');
define('HEADER_LOGO_IMAGE', 'logo.gif');

I wanna modify the info in above code.
Which file should I edit ?
What's the Relationship of the 2 header.php files?

The one in the classic folder is over-rides the other IF you are using the classic template. You can find out more about over-rides from the FAQ.

The fully correct answer to your question is that you shouldn't be editing either of them. You never edit the main language file, only ever a copy of it placed in a folder that has your template directories name. And that name shouldn't be classic as the contents of that folder will be over-written when you next upgrade. Instead you should be creating your own template with its own template folders even though the contents may be copies of what is in classic. There's more on this in the FAQ too.