Pull the infrared cover off. This will reveal the infrared LEDs and the grey plastic backup battery tray. Pull the small tab with enough force to pull out the battery, but no more. The tray's tab tears very easily. The battery's type is CR 2032.
Replacement is the opposite of removal.
If the main batteries are too low to maintain memory, you will risk serious data corruption or loss. Basically, it's a Very Bad Idea. Don't do it. Main Batteries 3.1. Without main batteries, the computer is forced to overload its internal fission reactor, which supplies most of the current.