It's soldered down/integrated. There is a mini pci express slot that the WWAN connects to, but it's wired electrically to the USB2 bus, not the PCIe bus, so that doesn't help.
(I wouldn't be annoyed if it were possible to swap it out for larger storage somehow.)
Do you happen to know if it is integrated into the motherboard or if they are simply using a mini pci express card internally?