Carefully to start with...
Depending how its been stuck down, i would assume some type of 2 part epoxy, you may be able to get a stanley blade between the deck and hull and cut it out, once out you may even be able to pick the old glue off.
Using chemicals is a bit risky with ABS plastic to remove epoxy as they tend to melt or deform the plastic, we can only hope you were lucky and it was glued down using a hot glue gun in which case it a fun project for an hour peeling it off!
Other methods maybe to destroy the deck slightly by cutting it out of the hull using a dremel, maybe cut it out 5mm from hull edge to keep the hull in shape and use as a edge of the old deck to glue a new one too, a deck is not complicated to remake as long as you can get the shape, which depending if its a flush fit, is a simple matter of turning up the hull and drawing around the edge only some ply ( try using 1 or 2mm lite ply for a deck) and cutting out.
If you cut out the old one and stick a new one to the top of it, you are able to use some P38 (body filler) to cover the gaps or seam that's left if you want a flush fit.
Ok its not perfect, the deck will be up to a few mm higher but that's the best i got for you suggestion wise.
If its more complicated and inset with bulwarks then you can still make a new deck in a similar way just over size it and shape to fit to its surroundings.