Which image are you using?
What changes do you want to do?
Have you tried in /sys/bus/rpmsg/drivers/intel_mid_rpmsg?
If you are not able to do changes in /sys for the rpmsg you will need to do the changes while building the image, this is because the rpmsg framework is related with kernel drivers. Another way is to install/create an API for rpmsg messages:
I hope this helps,