update_metadata_by_mid( string $meta_type, int $meta_id, string $meta_value, string $meta_key = false )

function update_metadata_by_mid( $meta_type, $meta_id, $meta_value, $meta_key = false ) {
	global $wpdb;

	// Make sure everything is valid.
	if ( ! $meta_type || ! is_numeric( $meta_id ) || floor( $meta_id ) != $meta_id ) {
		return false;
	}

	$meta_id = intval( $meta_id );
	if ( $meta_id <= 0 ) {
		return false;
	}

	$table = _get_meta_table( $meta_type );
	if ( ! $table ) {
		return false;
	}

	$column = sanitize_key($meta_type . '_id');
	$id_column = 'user' == $meta_type ? 'umeta_id' : 'meta_id';

	// Fetch the meta and go on if it's found.
	if ( $meta = get_metadata_by_mid( $meta_type, $meta_id ) ) {
		$original_key = $meta->meta_key;
		$object_id = $meta->{$column};

		// If a new meta_key (last parameter) was specified, change the meta key,
		// otherwise use the original key in the update statement.
		if ( false === $meta_key ) {
			$meta_key = $original_key;
		} elseif ( ! is_string( $meta_key ) ) {
			return false;
		}

		$meta_subtype = get_object_subtype( $meta_type, $object_id );

		// Sanitize the meta
		$_meta_value = $meta_value;
		$meta_value  = sanitize_meta( $meta_key, $meta_value, $meta_type, $meta_subtype );
		$meta_value  = maybe_serialize( $meta_value );

		// Format the data query arguments.
		$data = array(
			'meta_key' => $meta_key,
			'meta_value' => $meta_value
		);

		// Format the where query arguments.
		$where = array();
		$where[$id_column] = $meta_id;

		/** This action is documented in wp-includes/meta.php */
		do_action( "update_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

		if ( 'post' == $meta_type ) {
			/** This action is documented in wp-includes/meta.php */
			do_action( 'update_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
		}

		// Run the update query, all fields in $data are %s, $where is a %d.
		$result = $wpdb->update( $table, $data, $where, '%s', '%d' );
		if ( ! $result )
			return false;

		// Clear the caches.
		wp_cache_delete($object_id, $meta_type . '_meta');

		/** This action is documented in wp-includes/meta.php */
		do_action( "updated_{$meta_type}_meta", $meta_id, $object_id, $meta_key, $_meta_value );

		if ( 'post' == $meta_type ) {
			/** This action is documented in wp-includes/meta.php */
			do_action( 'updated_postmeta', $meta_id, $object_id, $meta_key, $meta_value );
		}

		return true;
	}

	// And if the meta was not found.
	return false;
}