Before you can install BwPostman, the installation package has to be downloaded. At https://www.boldt-webservice.de/downloads/bwpostman.html you will find the download for the current version of BwPostman, at https://www.boldt-webservice.de/index.php/en/joomla-extensions/bwpostman.html You will find an overview of the downloads to BwPostman and the manuals as well.
In the back-end of Joomla!® you also find the download of BwPostman, namely in the web catalogue of the extensions.
Since version 2.0.0 as a package as a whole, which may be installed as is and that contains all free parts of BwPostman, as there are component and the above mentioned modules and plugins.
BwPostman serves the automatic update function in Joomla!®. That means, that it will be shown in the back-end as soon as a new version of BwPostman component or modules is available. When the Joomla!®-integrated plugin Quick Icon - Joomla!® Update Notification is activated, you can see this when you enter back-end and the control panel will be shown. A click at the button Updates are available will show you the available updates, even so the updates for BwPostman.
When the plugin is not activated, you can look for updates in the back-end in the menu Extensions → Extension Manager in sub menu Update (clear cache before if necessary) and install it.