I would be very surprised if there was no CMOS battery on the motherboard. I know Amiga used rechargeable batteries but know of no Windows PC that has ever used them.
There are two options both involve opening the case so disconnect from the mains power first.
Look at the motherboard for a battery. Generally found near the bottom and it looks like a small silver disk.
You could also examine the motherboard for its identity, usually a manufacturer and part number of some sort on a clear area of the board. Having found this do a search for it online and look for a picture or battery replacement instructions.