First just copy the whole update method...
Then remove all the clutter that can not be true if you know that the item is AgedBrie...
Don't forget to remove all the AgedBrie stuff from the origional update method in Item...
Did the same for the Item with Sulfuras in the name...
Copied the update method from Items to the created Sulfuras class en started cleaning it up. It left me with an empty method :-)
Removed all the Sulfuras stuff from the update method of Item.
Did the same for the Item with Backstage pass in the name...
Copied the update method from Items to the created BackstagePass class en started cleaning it up.
Removed all the BackstagePass stuff from the update method of Item.

Here we have the first Testcase change because the aRandomBackstagePass() test prepare method now needs to return BackstagePass i.s.o. Item.