Post mobo model (if you know; ex: Iwill KK266) and I'll show you the pic with battery location.
Otherwise.. why do you need it replaced? you've noticed that your system clock has gone haywire? Open up your box.. look at mobo.. spot the
shiny circle
It's usually supported by a push-down thingie.. and one a part of the bed, that the battery's in, there should have a depressed area.. use it....
Take out the offending part.. run down to your local
Radio Shack.. they'll (hopefully) give you a compatable replacement.
You could also read mobo's manual about specs for the system battery.. or seach for info on battery's info.. it'll tell you the needed specs.