There does not appear to be a specific BIOS.
Although the hardware is based on a PC architecture system-on-a-chip the firmware would appear to be a dedicated application without an explicit split between applications, an OS and an industry standard BIOS.
Updated Firmware is available through the Axel website although you have to register first. Unusually you don't download it. Having selected what you want it is then emailed to you.
The latest firmware file - including support for USB drives - was 580KB in size.