As shown here use your fingers to gently pull the battery-free. Replace that old battery with a brand new Acer Aspire One replacement battery from Laptop Battery Express. Slide the battery pack out of the battery bay.
Unscrew them and carefully lift the cover.
Update the bios as follows. To reset the CMOS battery on a Acer 5315 is to short out the CMOS battery on boot up by using a screwdriver on both leads of the battery. It should come right off easily. He inserts the battery easily.
