On the side of the main body is a decalcifier cap. ENSURE THE IRON IS COOL before removing this cap. Unscrew the cap and flush the tank out with a decalcifier.
If you live in a hard water area it may be better to add distilled water to the tank instead of tap water to prevent lime scale build up. If you have a condensing tumble dryer then use the water that you collect in the dryer bottle. It is excellent for your iron as it is free from harmful impurities!