An user with an Exchange mailbox needs to change their password. For POA-managed Hosted Exchange mailboxes ability to change a password in OWA is disabled to avoid problem with passwords in POA database and in Active Directory being not in sync.
User has to change their password in POA Control Panel. Changing password there will affect all user's services, including Exchange.
This can be done using the 'Forgot password' link on POA login page which will send a reset password link to user's email (details are available here: http://download.pa.parallels.com/poa/5.4/doc/index.htm?fileName=52263.htm) or in service user's control panel (MyCP). To change password in MyCP, user has to click the 'Login' link in the 'User Account' tab, then click the 'Change Password' button: