How to cast an object class to another object class?

I tried to change some data in an object array class,

Diving test = null;
for (int x=0; x.lodger.length;x++){
   if (lodger[x] instanceof Snorkelling){
        test = (Diving) lodger[x];
        test.setName("Julian"); } 

But it come out error, said that, class Snorkelling cannot be change to class Diving.

I want to change that object into another object class with given set of data. But how? Do I need to use interfaces?