For doing so, login to your raspberry py with putty or another SSH client.
Now you have to change one value in the noobs.conf. To be able to access this config file you have to do some stepps:
Make directory to mount
mount noobs partition
sudo mount /dev/mmcblk0p3 tmp/noobs
look which os is on which partition
This part is interesting:
"name" : "RISC OS",
"partitions" : [
the mmcblk0p 5 is important
edit the noobs.conf with e.g. nano and change the value
default_partition_to_bootto the partition number
sudo nano noobs.conf
Now finally perform a reboot to chage to the other os
If you want, you can make a shell script which changes the value and performs a reboot. That would make it easier, if you want to do this often.