How can I clean out the water tank? Mine is generating some brown substanc?

My Stirella Simax 4580D is approximately 5-6 years old. In this model I understand one should use half tap water and half distilled water. This has been fine till a few months ago when it started to produce some brownish substance when the steam was turned on.
Please could you inform me how to "wash out" the water tank?
Thank you