Using a web browser, log on to the mission hub at Mission Hub - Litchi
Once you're logged in, click on 'Missions' in the lower left corner, then click on 'open'.
Then delete the missions you no longer want to sync to your device.
To delete a Litchi Mission on your iOS or Android device select the "Open" folder, do not load the mission you want deleted, simply swipe to the left. You should see the option to delete the mission.