How does a user change their password?

I have been using OS X server to provide, among other things, centralised user accounts, such that any user can logon to any mac on the network by providing mobile accounts. However, if a user wants to change their password, this only appears to affect the local (cached) copy of the account and doesn't appear to change it centrally. i.e. use System Preferences-> Accounts

How does a user change their 'network' password in OS X Server?