Unfortunately, you cannot use the webmail client to change your password. You must use a separate URL to modify both your password and your forwarding information. If you use the forward config within the webmail client, it will not work.
First, go to https://mail.trekfics.net/postfixadmin/users/, and use your current credentials to login.
Once logged in, you may click on one of two locations to change your password. Click on either to move to the change password screen.
From this screen above, you can enter your current password, and then type and re-type your new password. Click on Change Password to submit the changes.
NOTE: If you are using a POP or IMAP client, you will need to change the password to the new one in order to retrieve mail.