To manage your email accounts, use the administration interface at https://zimbra.ossds.net:7071 
Only users that have been designated as administrators for your domain can login to the administration interface.  

The manage accounts pane is the first thing you will see when you login. Select the account you wish to change the password for and click on the Change Password button above the account listing. 

When you change a user's password, make sure to update the password on all email clients that access the account.  This includes iPhones, iPads, Android devices, Outlook and Zimbra Desktop.  Not updating the password on every email client will result in the account being locked.

When you are in the admin interface you will see an account that is galsync@yourdomain.com. This is a system account that does not count toward your email account limit and you are not charged for it. Do not try to modify it in any way. Modifying it will cause problems.