Method to change value in a key for a min heap

How would you write a method to change the value of a min heap where bool changeKey(int oldKey, int newKey). The keys are unique, no duplicate keys are permitted. If there is a key in the heap with value oldKey, and no existing entry with key newKey, it will change it to newKey, keeping the the heap properties and returning true. If there is no key in the heap with value oldKey, or an existing entry has value newKey, then it will take no action and return false. Also an auxiliary map is used to store the locations of the keys in an array. I know how to write methods for insertion but haven’t seen any implementations of this.