The battery compartment is now visible on the exposed side where the grip was attached.

Using your fingernail or a spudger, pull the battery release switch to the left to release the battery.

The battery is spring loaded, so it should pop out, letting you grip it and pull it out of the device.

If the battery won't come out, it could be swollen. Leave it there - you will have an opportunity to push it out from the inside later.

Unfortunately my mouse does not have a battery in it (it got swollen a long while ago and I didn't replace it), so you'll have to imagine there's a rectangle-shaped battery in there 😅