|You need to read the manual for the computer to do that, if only to find the location of the jumper.|
Basically you turn the computer off, remove the power cored, open the case and locate the CMOS reset jumper. This is often near the CMOS battery. Take the jumper of the pins it is on and replace it on the empty pin and the middle pin. Wait a couple of seconds and put the jumper back where it came from.
Replace the case, apply power and re-boot.