Starting at the top of the phone near the antenna, use a screwdriver or other small tool, maybe even fingernails, to pry the rear panel off.

There are 4 places where the panel snaps together to the main part of the phone.

By systematically prying up the edges of the back panel, moving about a quarter of an inch, and prying again, the panel will begin to lift off.

Don't be afraid to damage anything, the panel requires a bit of force to detach.