Hello there and welcome to fixya
This is a very common issue with these desktop units the cmos battery will only last from 2 t0 5 years depending on how it is used.
This battery sits on the motherboard so you will want to first unplug the computer then remove the outer cover to expose the internal parts then look for a coin sized silver battery that looks like a watch battery then replace the battery with a new one
radio shack has these then power up the computer and you will have to reset the time and date ok you will get a error about this. here is what the battery looks like
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