create a blank php file and paste the following:
<?
phpinfo();
?>
upload it to your server and run it. you should see a whole list of info about the server's PHP installation including (near the top) the path to the configuration file (php.ini). once you have the path create another script like this:
<?
$copy = copy('/path/to/source/php.ini', '/path/to/dest/php.ini');
if ($copy) {echo 'copy OK';} else {echo 'copy FAILED';}
?>
obviously substitute /path/to/source/php.ini with the complete path to the serverwide php.ini and /path/to/dest/php.ini to your document root. upload it and run it. assuming your copy worked you can then edit your copy of php.ini with whatever setting you want.