Its probably been on there and possible the orignal. The torx is seized. You can used penetrating oil. The rachet with adapter will give you the extra leverage to get it off or let the penetrating oil do its thing. I presume you are trying to use a screw driver type of torx. You can rap a rag around the handle to save your hands. Once it starts turning its come off easy.
Replace the rotor by lifting off the old one and putting the new one on the way it came off.