The battery is a silver, round, flat thing 2 cm in diameter, like a coin. It is usually a type CR2032. There is a little clip on one side which you push back with a pen cap etc, and the battery will pop out. When you take it out, the CMOS will drain in a few moments or a few minutes and your bios will reset to default. This could reset the date and time to the year 2002 or so. In other words dont take it out until you have the new one ready, unless you know what you are doing grubbing around in the bios.