the text above the Label and the header

Above the label and header? What do you mean by that?

If you’re meaning to change the header or title itself, the one that says “Leave a reply”, then you can add this code on your child-theme’s functions.php

function custom_comment_form_defaults( $defaults ) {
	$defaults['title_reply'] = 'Your New Text';
	return $defaults;
}
add_filter( 'comment_form_defaults', 'custom_comment_form_defaults' );