The Zen Zone can be upgraded by Zen Zone Upgrade Kit item. To upgrade the Zen Zone, use the Zen Zone Upgrade item on the Zen Zone that is already situated in your city. Its upgraded version will then be placed directly on the same spot in your city and the production timer will not be reset. Note that the upgraded building will be from your current era, even if the old one was from one of the earlier ages.
Every 24 hours the Zen Zone will produce the following resources. Base resources will always be produced. Bonus resources are determined by how many other unique Cherry Garden Set buildings it is connected to.