The indoor unit is dirty. You must disassemble it. take it apart and wash it, if possible with a pressure washer and some degreasing product. If it is split you must store the refrigerant in the outer unit. loosen the connections, both plumbing and electrical, and do what was mentioned before. but you must have the right tools for the job if it is wall or square, take it apart, take it apart carefully and wash it ....