Change username on custom php page.

[Shalom.]

hello, i just make a new php page that allowed users to change their usernames.
i use this code:
$db->query_read("UPDATE user SET username='$newusername' where userid=$userid");
but it's sucks because in the fromdisplay i still see the old username.
i tried to find the code that used on the admincp but i didn't find it..
please help, thanks.