There are instructions to replace the thermostat at Volvospeed.com Its quite easy to do but you need the right tools to do it. Otherwise, you could strip the torx screw that holds the thermostat housing together. One other thing, have you replaced your air filter? Check out the link below. They have all sorts of DIY instructions on repair at Volvospeed. Picture below shows you the thermostat housing which you need to open to replace the thermostat. Good luck.

http://volvospeed.com/volvo_performance.php