How to clean the EVAPORATOR of C4 picasso(B78)

My C4 picasso(B78) blows very smelly air like sour and foul odor when A/C is on. I already change the air conditioner's filter. But it worked for just few days. Someone told me that the surface of the EVAPORATOR is needed to be cleaned.

Anyone have cleaned the surface of evaporator before? could share the experience? Or show me the structure of the air conditioner. I could try by myself based on that. Thanks in advance!