There should be a small slit in the case just enough for you to get a fingernail in to remove the back - that is, if it's supposed to be removeable. The battery will probably be screwed into the case or the main circuit board.
Before doing all that, unplug the mains supply to the device and press the power button for about ten seconds. That might remove all what we call the flea power; the small amount of electricity remaining in the device. If that fails, try holding down the Volume Up button while pressing the power button.
That bypasses the Android system and shows a menu of options. Use the Volume buttons to scroll and the power button to select an option and I suggest you take the "Clear caché" choice.