[Adrian Schneider]

You could run the query:
[sql]UPDATE user
SET usergroupid = 6
WHERE userid = 1
LIMIT 1[/sql]
This is assuming your are userID 1.

If you don't have SQL access, create a file and run it from the vBulletin directory:

PHP Code:

<?php

require_once('global.php');
$db->query_write("
    UPDATE user
    SET " . TABLE_PREFIX . "usergroupid = 6
    WHERE userid = 1
    LIMIT 1  
");

?>

or for 3.0.x.

PHP Code:

<?php

require_once('global.php');
$DB_site->query("
    UPDATE " . TABLE_PREFIX . "user
    SET usergroupid = 6
    WHERE userid = 1
    LIMIT 1  
");

?>