RPi3 Change Berryboot Default System By SSH

Step

1. Use sudo mount /dev/mmcblk0p2 /mnt to mount raw storage

2. Use ls /mnt/images to check what the full file name of the operating you would like to run

3. Use sudo su to change user into root

4. Write out the desired image file's name to "runonce":
   echo [filename_of_operating_system].img[memsplit amount] > /mnt/data/runonce
   For example: echo Raspbian.img128 > /mnt/data/runonce

5. Reboot

Note

That this will only change the booted OS for the next reboot. You could probably change the default by changing the 4th step to the following:
echo -n [filename_of_operating_system].img[memsplit amount] > /mnt/data/default