Remove the battery by inserting the spudger under the battery and gently pulling it up from the device.

Continue to push the spudger towards the center of the compartment holding the battery, and gently pry the battery away from the device.

In this image, the metal spudger is used. However, you should NOT use a metal spudger to remove the battery. Use the plastic spudger instead, as this is less likely to puncture the battery.

In these images, both circuit boards are removed, but this isn't necessary to remove the battery.