How to clean the EVAPORATOR of C4 picasso(B78)

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.