On Mission Hub i just use the command on the list of missions to erase the map with mission on it then save it. It is a symbol of a trash can under the heading "delete". On the Litchi app I have been using the "X" to clear the map of the mission and save it. It'll show up[ as a blank mission. I sometimes give a temporary name to it starting with "Z" so it ends up at the end of the list, out of the way. Probably a better way but that works for me for now.
Good luck
Jim
WA5TEF