The Upgrade can be upgraded by Upgrade Upgrade Kit item. To upgrade the Upgrade, use the Upgrade Upgrade item on the Upgrade 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.
For the Image to show, the image filename of the upgrade item needs to be of the following format