The battery requires some persuasion to come out. Lots of adhesive, and a tight fit inside the rigid black plastic piece. It is definitely not designed to be replaceable.

On one side - Li-polymer, 0.74 Wh, 4.2V. AEC 8376.

On the other side - 200 mAh, 3.7 V. AEC 531236.

0.2 Ah at 3.7 V would agree with 0.74 Wh, so it's not clear what the 4.2V marking refers to.