Dealing with the numpad is both tricky and requires force. I am separating the disassembly and reassembly steps as they require different techniques. This step is for disassembly only.

Hold the numpad casing and bottom casing together at the rear end.

While holding the back end together, gently pull the front of the numpad casing away until it won't gently pull away any further.

Now carefully pull the the numpad casing off of the bottom casing from the rear.

The two should come apart effortlessly at this point, but as I mention in one of the re-assembly steps, I broke the casing a little at the back when attempting this before so it may require more care and a slightly different approach.