M300/ 350 altitude change during mapping mission

[uav_operator]

Does anyone with M300/ 350 experience know the best way to adjust altitude during a terrain following survey mission (either with the Pilot app or 3rd party apps)? For example, I plan a mission to follow terrain at 20 meters AGL and during the mission I want to add 5 meters to now follow terrain at 25m AGL, can this be done? Stopping and re-uploading the mission is not an option. It must be done on-the-fly simply by bumping the altitude up or down as needed seamlessly. Thanks