King Charles Spaniel's are not supposed to have an overbite or an underbite according to AKC standards, however, puppies are known to start off with these, and theire teeth will set over time giving them an even bite.
If your dog is over seven months old, and the overbite/underbite has not dissapeared and self corrected, you should get in touch with the breeder you bought the puppy from and ask for more information on the parents and potential health problems as this could be a genetic fault in the dog's make-up.