updating elements of an array which is part of a composite type?

How do I update the elements of an array when the array is part of a 
composite type?

If I type:

  update mytable set (mytype).myarray[1]='whatever' where ...;

I get :

ERROR: syntax error at or near "(" at character ...

indicating that the first bracket is not acceptable.

I have no trouble selecting via:

   select (mytype).myarray[1] from mytable;

Thanks for any help.
Celia McInnis