To remove the rear casing, use a spudger to leverage the sides of the metal frame at the bottom loose. If the spudger is in danger of breaking, it is safe to use a screwdriver.

With that, the entire inner assembly should be separated from the rear casing. Carefully remove it.

During reassembly, when re-inserting the core assembly into the plastic casing, remember not to let the small red wire (visible in the third image) catch onto the small black hook on the casing (green)! The wire must be held down so as not to be hooked.