Place the can upside down between the jaws of a bench vise, put a rag over the top, and hammer a nail down to pierce a hole in the bottom, through the rag. Once the pressure is released, you can just use a can opener to make a larger opening and empty the content. Use normal safety precautions and wear safety glasses, and do this in a well-ventilated area in case the propellant is flammable. I did it once to get some lubricant out of a nearly-full can, and nothing whatsoever happened when I pierced the hole through the can and I was able to retrieve the whole content. Again, use caution and good safety measures just in case.