How do I stop my index.php from displaying "We're currrently updating our system and will be back up shortly. Sorry for the inconvenience."

Below are the contents of index.php:

<?php

define('gpdebug',true);
//define('gp_indexphp',false);
//define('gptesting',true);
require_once('./include/main.php');

If I can't change the file directly, how can I get back into admin to make changes to what is there?

Thank you!