You may find the below method in philips webby.
You should descale the sterilizer as follows:
- Always unplug the appliance and wait until it has cooled down completely before descaling.
- Pour 80ml/2.7oz of white vinegar (5% acetic acid) and 20ml/0.7oz of water into the water reservoir.
- Leave the solution in the water reservoir until the scale has dissolved.
- Empty the water reservoir and rinse the heating plate with clear water. Empty, then wipe the plate with a damp cloth. Note: Do not rinse under a tap!