Use sudo mount /dev/mmcblk0p2 /mnt to mount raw storage
Use ls /mnt/images to check what the full file name of the operating you would like to run
Use sudo su to change user into root
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
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