What you have is not a "microcontroller" but rather an Embedded Linux System built around an Arm Cortex A8 "System on a Chip" (SOC)
Pressing space at startup is typically the behavior of a human-interactive bootloader, for example the very common Das U-Boot or proprietary alternatives of similar functionality. These are much like the startup / configuration menu of a desktop PC, in that they are small programs that run first on startup and allow selecting the source and configuration flags of the primary operating system to be run, in your case typically a Debian Linux. In addition to booting an operating system from removable media, in the case of more compact systems they may be configured with the capability to copy a new image to internal storage - though Debian systems are often large and complex enough that a (perhaps smaller) instance of the OS is booted and used to do the full copy.
Generally speaking the communication channel used is a serial port to be connected to a serial terminal. On the SoC itself this would be logic level (3v3 or quite possibly even lower) but on a finished product it may or may not be inverted and translated to RS232 levels.
You will probably need a serial terminal program and a suitable electrical interface - typically an adapter from your PC's USB port to either RS232 serial or logic level serial. You will also need to set an appropriate baud rate, though 115200 is quite common except for a few SOCs with serial timing issues that cause 57600 to be chosen instead (Additional settings of N81 with no flow control are nearly universal)
Ultimately the documentation for the product you are trying to use is hidden behind a registration portal. You'll probably need to jump through the necessary hoops to register, then study the documentation. This will help clarify if you will be interacting with U-Boot or some other bootloader, and if you will be doing the copy at that level, or if you will be allowing some instance of Linux to boot and then have that perform the copy to internal storage.