How do I legally revoke my name as my father's medical and durable power of attorney?

Is there a successor? Is so, let dad and that person know you are resigning. Did an attorney prepare the documents? Then let the attorney know. If no attorney contact the Area Agency on Aging to ask for guidance. Are you sure this is what you want to do? It could be very difficult to undo once done. Could put him in a situation that he would become a ward of the state taking away any possibility of you having any control or contact with him.