That depends on the motherboard. If, instead of having pins sticking out of the bottom of the processor, it has little pads, and these pads are soldered to a ball system, upgrading is impossible. On some other machines, you can use a screwdriver to take the processor off and put a new processor in, but you're still limited: your motherboard will only take certain processors, with a certain speed and pin configuration. On a Mac, the processor is soldered to the mainboard, so it is not upgradable.