Update Pixhawk Mission Plan Offboard in Real Time

I am trying to make real-time mission plan changes (i.e. change mission waypoints list) offboard. I know the Middleware deals with communication with a companion computer and allows for a uORB subscribe message. I believe I should be trying to update a uORB topic offboard mid-flight, but I am uncertain which one and how to do it via a MAVLink command. Any suggestions?