1. Select the Mission you no longer want to be unlocked by another Mission
2. Select the "Availability" header at the top of the Mission editor
3a. Click on "When another Mission is completed" and chose a different option to trigger the release of this Mission
Choose "When the Experience starts" if you want to set it back to the default criteria.
3b. You can also search for another Mission to replace the current releasing Mission from your experience
4. Click on "Save changes"
5. Now on the Mission page you will see the the two Missions are no longer connected