First you want to make sure you have a fully charged battery in your S2. Then, format a CF card in the camera. Put the CF card in a reader into a computer, then copy the firmware (the .S2 file) to the memory card. Don’t put it inside any folders, it goes in the root directory of the card. Once that’s done, put the card in the camera (while the camera is off) and turn the camera on while holding down the rear AF/AE lock button (located directly right of the viewfinder). The camera will recognize that the CF card has firmware on it, and will update. Takes a few minutes to complete the process.
Hope that helps!