Is has eMMC storage, which is a chip soldered to the board. Upgrading the onboard storage would require removal and re-soldering with a hot air rework station and possibly a board pre-heater. Not a DIY job for most people.
Furthermore, even if a larger eMMC chip were to be soldered on, there is no guarantee that the BIOS would recognize it. This upgrade would be experimental.
I recommend you use external USB for additional storage.