The aluminum backplate is glued to the device and you will need a good prying tool to be able the separate the back from the plastic holder.

The glue is holds all 4 sides together, the glue also runs vertically and down the middle.

The hardest part is to start somewhere on the backplate where there is a big enough gap (between the backplate and the device) to put a prying tool in without bending the aluminum back.

Start from one of the upper corners. Slowly and carefully remove the aluminum back plate with a prying tool that is durable enough and can fit between the backplate and the device.