Once the penetrating lubricant has soaked in for a few minutes use your 10mm socket and ratchet to loosen the three bolts that hold the cover on to the thermostat housing. When re-installing the nuts note that they should be torqued to 10nm / 7lb-ft

Once the three bolts are out you can separate the cover from the housing. It will still be held in place by the lower radiator hose but there will be plenty of room to access the thermostat.