Mostly tried this out. You'll need an 8GB+ (16GB+ prefered) microSD/SD and a USB keyboard.
1) If using Windows, download and extract ext2explorer:
https://sourceforge.net/projects/ext2read/
2) Go to to
http://lakka.tv.
3) Click Get and follow the steps to get the "Allwinner Orange Pi" image, "OrgangePi Lite" and write the image using the instructions provided from lakka.tv on the microSD/SD.
4) On the Super Retro-Cade, boot once to allow the auto-resize of storage (it'll take a while)
5) Back in WIndows, on one partition is "uEnv.txt". Use Notepad++ or similar and change the first line to read: "bootargs=console=ttyS0,115200 console=tty0 boot=/dev/mmcblk0p1 disk=/dev/mmcblk0p2 consoleblank=0 quiet tty retroarch=0" (ie add "tty retroarch=0" to the end) to enable a terminal and disable auto starting retroarch service
6) On the second line change "sun8i-h3-orangepi-lite" to "sun8i-h3-orangepi-pc-plus.dtb" -- this will allow the nand to be visible.
7) Use ext2explorer (run as administrator) to access the second partition and copy the firmware there.
8) SD back in the Super Retro-Cade, let it boot up. You should end up with a Lakka# prompt in the corner. It should auto mount the nand/emmc/whatever to /storage/roms/<different partition names>
9) With the USB keyboard connected, you'll need to umount all the nand partitoin stuff: "umount /dev/mmcblk2p*"; it'll complain about all the stuff not actually mounted.
10) You should already be in storage. If you type "ls" you should see your firmware listed.
11) If (10) is true, you should first backup your current firmware with "dd if=/dev/mmcblk2 of=firmware_backup.img bs=1M". When done, "ls -l firmware_backup.img" and it should be ~3.7GB big.
12) if (11) is true, you can try writing over a new firmware. This is the destructive bit, and not something I've verified (but it should work). If things go wrong, you should be able to follow step (14) to recover your Super Retro-Cade. Having said that, if you want to take the risk and presuming you named your firmware "firmware.img", use "dd if=firmware.img of=/dev/mmcblk2 bs=1M".
13) You might want to run "sync" first. But definitely run "shutdown -h now". You should see a "reboot: System halted". And that point, turn off the Super Retro-Cade, take out the SD card, and reboot and see if the firmware works.
14) If something went wrong and presuming you made a backup, power off the Super Retro-Cade, follow steps 8-10, and then do "dd if=firmware_backup.img of=/dev/mmcblk2 bs=1M".
I can't stress enough that I haven't verified any of this, but it should be as straight forward as the above. AFAIK, so long as you have a good firmware you should be able to boot from the SD slot and restore a firmware. Personally, I'm not interested in risking it.