Can't upcast interface to extended interface

Question

I have 2 interfaces

public interface A {
 method1();
 method2();
}

public interface B extends A {
  method3();
  method4();
  :
}

I am in need of converting (upcasting) B to A in one of class. But so far I could not figure out how do I do that.

Assume "methodWhichReturnB" is method which returns B.

public class C {

  public void method10() {
     A a = methodWhichReturnB; // din't work
     A aa = (A) methodWhichReturnB // din't work          
  }
}

I am getting compilation error "Inconvertible types; cannot cast B to A".

Is there way I can upcast B to A ?