Instructions for Updating an Existing Zulu Installation

If I'm an existing customer, how do I go about updating an existing Zulu installation with the latest upgrade?
Haven't found any procedure on the website that refers to that. By upgrade, I mean the newest version that supports Google Calendar as well as the past feature set.
Don't want to blow away anything critical in a mis-guided attempt to remain current;)
Haven't found any procedure on the website that refers to that. By upgrade, I mean the newest version that supports Google Calendar as well as the past feature set.
Don't want to blow away anything critical in a mis-guided attempt to remain current;)