If you want to change your postmaster email account or other email account's password, you can consider logging to you hosting control panel to change it, please follow the steps below to change it.
1. Login to your hosting control panel
2. Click on 'Emails'
3. Click on the Manage Users & Quotas under the "Actions" option.
4. Click on the Edit email user icon under the actions option besides to the email account which you would like to change the password.