1. Unplug the machine from the mains and remove the cover. Refer to your motherboard manual for the jumper that resets the CMOS to the factory defaults. You will normally have to set it to the reset position for a minute or two and then back to it's normal position.
If you don't have access to that information;
2. Locate the motherboard battery, and remove it CAREFULLY for a minute or two, then replace.
Note. For both of these procedures, it is essential you do not have any mains voltage to the machine. You can do this either by unplugging OR just turning off the wall socket. I prefer the later, as you still have an earth connection, which tends to disperse any static in your body.