A CMOS battery is there to keep the computer's clock running and possibly to save BIOS settings. If it is a desktop computer it is normally a CR2032 or CR 2016 button battery in a holder on the motherboard. I would recommend replacing it and it is not hard at all. Power down the computer, unplug it, remove the case and find the battery. There will be a little spring clip on one side to release the battery. Get a new one and pop it in. All done. When you boot up after replacement you will have to set the clock, and maybe BIOS settings. I doubt that you will need to mess with bios as newer computers store BIOS setting in non-volatile memory and it will stay even with a dead CMOS.
On a laptop it is probably a little harder because you will likely have to disassemble the laptop at least partially to access the battery. On a laptop it could be a battery on a pigtail plugged into a connector on the motherboard.