To gain access to the internals we need to remove the upper shell, to do this we need to remove two more screws, which are (unfortunately) located underneath the two main mouse buttons.

The left button is rather easy to remove, just apply light upwards pressure between the shell and the mouse button, you will be able to lift it up as it's on a hinge, and gently remove it from the hinge for better access to the screw

For the right button, apply pressure from multiple angles if the button doesn't come off like the left button. Try to apply pressure from the points marked by yellow arrows.

Once both buttons are removed, we can access the screws located in the red circle markers, use a Phillips screwdriver to remove them.