To the right of the MicroSD slot is a rectangular slab that’s secured by a screw. That’s your battery right there.
Taking it out should be familiar to anybody who’s ever taken out a cell phone battery before. The only exception is the aforementioned screw, which you’ll need to unscrew with a small Phillips screwdriver.
Once you’ve taken the screw out, put your finger on the crescent-shaped recess and pull out the battery. Easy-peasy, as they say.