To cancel a building’s production, simply click on the building to open its interface. You will then see the currently active production and its remaining time.
Supply Buildings
Open the building and click the red Cancel / X (if you play on mobile) button to stop the current production. You’ll be asked to confirm your choice. Once confirmed, the production will be canceled.
Goods Buildings
Open the goods building and click the red Cancel button. After confirming, the production will be stopped immediately.
Military Buildings
To cancel a unit currently in training, open the military building and click the red Cancel / X (if you play on mobile) button. A confirmation window will appear — confirm to cancel the training.