Flip it over

Here I stuck a screwdriver in the notch on the compartment cover. You have to use quite a bit of force to push the screwdriver in there.

Better to use a micro-screwdriver for this instead of what I did. It would probably slip in there easier.

Extreme caution should be taken during this step. Try not to suddenly push a metal screwdriver in there because the battery is right behind it. You don't want to puncture the battery.