The easiest way to clear the CMOS is to enter the BIOS setup utility and choose Reset BIOS Settings to their factory default levels.
The exact menu option in your particular motherboard's BIOS may differ but look for phrases like reset to default, factory default, clear BIOS, load setup defaults, etc. Every manufacturer seems to have their own way of wording it. The BIOS Settings option is usually located near the bottom of the screen, or at the end of your BIOS options, depending on how it's structured. If you're having trouble finding it, look close to where the Save or Save & Exit options are because they're usually around those.
Another way to clear CMOS is to reseat the CMOS battery. Start by making sure your computer is unplugged. Next, open up your computer's case if you're using a desktop PC, or find and open the small CMOS battery panel if you're using a tablet or laptop computer.
Finally, remove the CMOS battery for a few minutes and then put it back in. Close the case or battery panel and then plug in, or reattach the computer's main battery. By disconnecting and then reconnecting the CMOS battery, you remove the source of power that saves your computer's BIOS settings, resetting them to default.
Another way to clear the CMOS is to short the CLEAR CMOS jumper on your motherboard, assuming your motherboard has one. Make sure your computer is unplugged and then open it up. Look around your motherboard's surface for a jumper with the CLEAR CMOS label, which will be located on the motherboard and near the jumper. These jumpers are often located near the BIOS chip itself or next to the CMOS battery. Some other names by which you might see this jumper labeled include CLRPWD, PASSWORD, or even just CLEAR.
Do all these things VERY carefully, if you are not comfortable, then ask someone else for help, if need be.