the battery is a 3 volt cr2032 made by panasonic, replace with a similar battery to yours. it faces + down on my model, but it might not on yours, so you need to check before removing

to get the battery out, put your thumb under the battery, and push up while pulling the battery away from the motherboard (couldn't get a good picture of this)

If your battery goes ballistic and flies into the lower portion of you computer, you have to get it out or you risk permanently frying you desktop, use a magnet to remove the escaping battery

to reassemble you computer, follow these steps in reverse order