Batteries do not last forever, so this guide is to show you how to replace the battery.
Use a T-9 Torx screwdriver to unscrew the four screws on the back cover.
Using a spudger, gently pry apart the back case from the front case. It is easier to start on a side and work your way around the device.
Once the two covers are separated, use the tweezers, or even your fingers, to gently disconnect the two sets of wires that are connected to the motherboard from the back cover. You may have to wiggle the wires a little bit to get them to disconnect.
Using the spudger, gently pry out the battery.