If you are upgrading to Microsoft Dynamics GP 2018 from Microsoft Dynamics GP 2015 or 2016, there is no separate update process that needs to be completed for SmartPost 2018.  For SmartPost 2018, you will want to follow the steps in the Installation section on the machine where you want to run SmartPost.  This will install the SmartPost 2018 application into your Microsoft Dynamics GP 2018 installation and the update will be completed automatically in that process.