Updating a table column

This question has been discussed many times but I'm missing something. I'm trying to update the LastName field

of a table with a value in BeforeShow. I've been able to use $xt>assign to change the $message variable. I thought

that I could also change a table this way too.
example:
$_SESSION["$lastnname"] = "Smith";

$xt->assign("LastName", $lastname);
Should I be using the SQL UPDATE instead?
Thanks
Al