So I checked mine out, the XC90 is a little different than others but same general theory. towards the back you will see a tab, looks like three sides of a square, that is a tab/clip that holds the back in place. I wiggled mine to get it free but you may need to put a flat head into the top and bend it out slightly. While doing that lift up on the back, that will free that half.
I didn't go through with the front, but it looks like there is just a hex or star (torx) screw holding it in place from behind, move the seat all the back and up and you should be able to get to it no problem, installation is of course opposite of removal.
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