If you want to restore the BIOS to its default settings, you can find the 3-pin socket on the motherboard labeled clr_cmos. Disconnect the computer power cord from the wall. Move the jumper so it spans the middle pin and the pin that was not spanned originally. Leave it there for about 30 seconds, then move it back. Plug the power back in, and power-on the system. Press the correct key to access the BIOS (usually the Del key or maybe F2 key), and make any changes you need for your system.
Another way to restore the BIOS settings to the default is to unplug the power cord from the wall, and remove the circular silver battery from the board. Let it sit at least 5 minutes, then re-install the battery, plug-in the power cord, and power-on the system.