Change Twenty Twelve’s comment form in a child theme

You can use filters in custom functions to modify the default output of the comments form via your child themes functions file.

Modify the comment form “Leave a reply” text.

add_filter( 'comment_form_defaults', 'wpsites_modify_comment_form_title' );
function wpsites_modify_comment_form_title( $defaults ) {
$defaults['title_reply'] = __( 'Leave a Comment' );
return $defaults;
}

Remove The Website URL Field From Comment Form

add_filter('comment_form_default_fields', 'wpsites_remove_website_url_field');
function wpsites_remove_website_url_field($fields) {
if(isset($fields['url']))
unset($fields['url']);
return $fields;
}