The plastic button cover is attached to the black plastic disc (with the four screws) with thin double sided tape. You need to use a tool that looks like a guitar puck to separate the bond. Some very gentle heat would help but because as the button cover can easily warp. Slide the puck between the two parts and work around until the tape bond it broken.
The battery tray on the reverse side can then be removed by gently straightening the four IR LEDs.
