How to dismantle stem for a new one?

Tris Two types:

if it's an expander type; unscrew the top bolt til it's sticking out about 1cm, then wack it with a hammer (not too hard), that forces the expander out the bottom of the stem - you should now be able to remove it.

if it's a wedge type, then it may have become stuck in there (they should come loose straight away). Turn it upside-down spray some Plus gas, WD40, etc, leave it over night, then try again.

Put a block of wood ontop of the allen bolt though. You don't wanna scrath anything up.