Maybe it is us, but we have not found ajn easy way to start /plan a different mission when an other mission is already open in the app.
A restart and 'clear state' is what we do at the moment. Please add in the menu a 'new' mission option, with the option to save the current when starting a new one.
You simply can deselect the mission first (hit anywhere on the 2d map to deselect the mission). After that you can plan another mission.