I have an inventory program written to include an array and a method to calculate total cost for all inventory items entered. I now have to include a subclass that overrides
You simply declare a method with the same signature as the method in the parent class. So yours would look like:
package inventory3;
public class ItemDetails extends Items {
    private String Name;
    private double pNumber, Units, Price;
    public ItemDetails(String Name, double pNumber, double Units, double Price) {
        this.Name = Name;
        this.pNumber = pNumber;
        this.Units = Units;
        this.Price = Price;
    }
    // getters and setters....
    // The @Override is optional, but recommended.
    @Override
    public double calculateTotalPrice() {
        return Units * Price * 1.05; // From my understanding this is what you want to do
    }
}