user properties still showing old firstname in AD. how do i change it ?

hi,
in windows server active directory i created a user:
 firstname: joan
last name: t
display name : joan t

then later i change the first name:
firstname:jo
last name: t
display name: jo t

but still when i open the user properties: its shows "joan t" at top and in outlook address books, it still shows as joan t
have attached a pic of it to make it more understandable
how do i change this. ? any ideas ?