By default users are not allowed to change their mailbox/account password.

You need to define a new privilege in the group storing your users.

• Click on manage organization
• select your organization
• Click on Manage Users Group  in the organization selected.

 

• Choose you group name in the "Select group" list. 
• After choosing the group, click on the Privileges option

 

• A new windows appears and show you privileges you can set for this group.
• Choose the "Users allow" tab
• Change to red the "Change password" section
• click on submit.