Run it on its hottest setting with nothing in it. The newer units have a special cycle for cleaning and use 175 degree water.
By using very little soap in mine, I have never had to clean it. I also always leave the door open after each load. It is the soap residue that tends to grow mold, so if little soap is used, no residue will build up. One teaspoon to 1 tablespoon of soap gets all my clothes clean.