When the iron is cooled down unscrew the decalsifer cap as shown:
Add a decalsifier or iron cleaning agent into the tank and flush out with water to remove the limescale build up. Then re fit the cap, ensure it is tight to prevent any water leakage.
In hard water areas it is advised to do this once per month or avoid tap water by using special water for irons that can be purchased from the cleaning section of your local super market. Alternatively, if you have a condensing tumble dryer use the water from the collection tank. It's perfect for your iron as it is de ionised.
In soft water areas it will be best to decalcify the tank twice per year.