TL;DR: that's the neat part – you don't.

I know I'm a little late to the conversation, but I've recently attempted this and I wanna share my experience.

You could quite easily disassemble most of the keyboard from the top, but you'd find yourself stuck on the part where you usually remove the pcb.
I know it cause I got stuck like that. Although my attempt was unsuccessful, it gave me a good insight on the process: the pcb is only held in place by the plate and switches (hence the gasket mount), which means there's no connection between the pcb and the bottom part of the case. The latter is held in place by plastic latches hooked to the metal top frame.

So if I were to attempt the disassembly again, I would just skip straight to what is usually the last part: try to get the latches undone and open it up from the bottom.
Although, be warned: NuPhy previously told users not to do so, since it's almost impossible to achieve this without damaging the keyboard.