You don't need to disconnect the laptop's battery if you are only replacing the back panel: only perform this step if you will be handling or replacing the electrical components of the computer.

Disconnect the battery cable from the system board by pulling up on the battery connector (#1).

Turn the computer back over and open the display to the point where you can press the power button.

Press and hold the power button for 4 seconds to ground the system board so that you can safely remove components such as the RAM.