As you pull apart the narrow end, the other round end will also separate. There are some clips holding the top and base together which can be eased apart by moving the white top from side to side. Also, you can pull the grey plastic away from the white plastic to help the clips become free.

When the two parts are separated, do not let the rubber keypad separate from the front white plastic, and make sure the small red plastic on/off button stays in place. If either does happen, they can be easily replaced.