if you pause the video @ 3:21 you can see a little latch on the bottom side of the compartment. use your finger nail (if you have one) and slide it to the left.
Came here to find the battery size. Winced at the screwdriver destruction when you can open the back by just rotating the back using the tab you used to lift it out.
@@MrWhatYT You did it in a wrong way, there is a plastic protrusion on the cover to twist the cover and open it. That notch you did use to twist the cover is just an lock/unlock arrow :).
