The Face of the Ancient is a special building and is one of two unique decorations released exclusively for the Guild Expeditions. It can be obtained as a reward for a successful Guild Expedition encounter or found in a relic.
Together with Gate of the Sun God, it was one of the best happiness providing buildings available in the game, providing much more happiness per tile than premium decorations and even premium cultural buildings. However, the game update 1.95 seriously nerfed the happiness value - it now provides only 56% of the original happiness value, thus making it only a bit better than regular decorations and worse than premium ones.
The Face of the Ancient can be upgraded by Face of the Ancient Upgrade Kit item. To upgrade the Face of the Ancient, use the Face of the Ancient Upgrade item on the Face of the Ancient 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.