Yes it is a desk top PC. Time keeps either stopping or goes very slowly. If I have not used the PC for a week or two when I switch on the date and time are not correct. If I need a new battery how and where will I find it. Do you know what type of battery it would be or do they vary?
As Fruit Bat /\0/\' link above. Unfortunately you'll need to find out how to open your PC's case. Google for that. Remember to unplug mains power supply before attempting it.
Some info on you tube to replace cmos battery and select more than the one linked, cmos