Hi;

open this file:

catalog/language/english/shipping/flat.php

find this code (ther is only 2 lines):

$_['text_title']       = 'Flat Rate';
$_['text_description'] = 'Flat Shipping Rate';

change it like this:

$_['text_title']       = 'Delivery';
$_['text_description'] = 'Delivery Rate';

for the others languages open this file and do the same steps:

catalog/language/YOUR LANGUAGE/shipping/flat.php

Good luck

Sam