Change the featured image metabox title text

/*
 * Change the featured image metabox title text
 */
function as_change_featured_image_metabox_title() {
	remove_meta_box( 'postimagediv', 'YOUR_POST_TYPE', 'side' );
	add_meta_box( 'postimagediv', __( 'Header Background Image', 'as' ), 'post_thumbnail_meta_box', 'YOUR_POST_TYPE', 'side' );
}
add_action('do_meta_boxes', 'as_change_featured_image_metabox_title' );


/*
 * Change the featured image metabox link text
 *
 * @param  string $content Featured image link text
 * @return string $content Featured image link text, filtered
 */
function as_change_featured_image_text( $content ) {

	if ( 'YOUR_POST_TYPE' === get_post_type() ) {
		$content = str_replace( 'Set featured image', __( 'Add Header Background Image', 'as' ), $content );
		$content = str_replace( 'Remove featured image', __( 'Remove Header Background Image', 'as' ), $content );
	}

	return $content;
}
add_filter( 'admin_post_thumbnail_html', 'as_change_featured_image_text' );