How to modify a foreach iteration variable from within foreach loop?

When I try to do this...

Item[,] array = new Item[w, h];  // Two dimensional array of class Item, 
                                 //   w, h are unknown at compile time.
foreach(var item in array)
{
    item = new Item();
}

...I get Cannot assign to 'item' because it is a 'foreach iteration variable'.

Still, I'd like to do that.

The idea is to assign default Item class values to existing item.