take out battery, unplug laptop from charger
flip over, removed 6 screws from bottom of laptop
flip over, open laptop and put screen as far back as possible.
Take out the keyboard, top of keyboard has tabs that can be compressed to release it. (between the keys and the case.)
unhook ribbon connector from its plug in on system board.
take a flat piece of plastic like a guitar pick and run it along the seam all the way around the case where it connects to the bottom part. Lift front of top case.
Look down roughly in the front under case below where the B would be located on the keyboard. Remove the battery you see, go get new battery, put new battery in.
snap case back together, re attach the ribbon cable for the keyboard, snap keyboard back in.
shut system.
replace screws in back of system.
put battery back in, plug back in to charger. Reset the clock on the system.
enjoy for another 5 or ten years.