To change your email password you'll have to enter the Web Mail and then select "Options" from the menu at the top and click on "Change Password".

You'll be requested:
the old password
the new password
and re-enter the new password (Verify password).

Once you've entered the new data click on "Submit" to confirm or "Cancel" to cancel the operation.

To be able to access your WebMail services again after the password change, you'll have to log-in again.