Nothing complicated: Open your functions.php file, paste the code below in it. Save the file, and you’re done!
add_filter('gettext', 'change_post_to_article');
add_filter('ngettext', 'change_post_to_article');
function change_post_to_article($translated) {
$translated = str_ireplace('Post', 'Article', $translated);
return $translated;
}
Thanks to Smashing Magazine for the cool tip!