Are you sure its the heater? That's in the cabin so unless its missing a hose clamp or has a hole in the pipe in the engine bay, it shouldn't be spewing steam in from the tunnel area.
That said, its not a difficult job to do, but you do have to remove a lot of stuff to get it out - all the dash and interior cross brace have to come out. Just make sure you get all the screws before you pull the bits out and take pics of how things go back together.
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