if you want to update a mysql database row you can use the following query:

lets say i have a webform and the user can change their password:
$username = 'eric';
$password = 'passwd';

$sql = "UPDATE users SET password='$password' WHERE username='$username');


the above query says: update the table users on field called password with the value equals to passwd where the usarname field is equals to eric

done

hope this helps