wp-excerpt-change.php

<?php
// Plugin Name: Example. Change "Excerpt" meta box heading

namespace KDL\excerpt_change;

function filter_excerpt( $translations, $text, $domain ) {
	if ( $text == 'Excerpt' ) {
		return "New Excerpt";
	}

	return $translations;
}

// hook in as close as possible, in this case fake an action
add_filter( 'post_updated_messages', function($i) {
	add_filter( 'gettext', __NAMESPACE__ . '\\filter_excerpt', 10, 3 );
	return $i;
});

// and clean up as soon as possible
add_action( 'dbx_post_advanced', function() {
	remove_filter( 'gettext', __NAMESPACE__ . '\\filter_excerpt', 10, 3 );
});