[nico_swd]

You need the old username or the userid, and the new name in order to get this to work. What does $username hold? The new name or the old one? Your query needs to contain two different values. It can either be the userid, or the old username. Something like this.

PHP Code:

$db->query_write("UPDATE review_users
    SET username='". $username ."' WHERE username='". $old_username ."'"); 


Or

PHP Code:

$db->query_write("UPDATE review_users
    SET username='". $username ."' WHERE userid = ". $userid);