How To Remove Namespace From Existing XML Subtr

I'm facing an issue with removing namespace from existing xml subtree.

For Example: I have a table with 1 field - data type xmltype. One entry is holding this xml:

<Item xmlns="http://www.w3.org/2001/XMLSchema-instance">
<Box>Blaff</Box>
<Door>Steal</Door>
<Chair>Wood</Chair>
</Item>

I would like to remove the namespace - i.e.:

<Item>
<Box>Black-box</Box>
<Door>Steal</Door>
<Chair>Wood</Chair>
</Item>

I tried using the UPDATE() function with no luck.