The Helter Skelter can be upgraded by Helter Skelter Upgrade Kit item. To upgrade the Helter Skelter, use the Helter Skelter Upgrade item on the Helter Skelter 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.
Helter Skelter will produce the following effects. Base effects will always be produced. Bonus effects are determined by how many other unique Horror Circus Set buildings it is connected to.