You can only delete a mission that you created yourself, to do so please follow the steps mentioned below.
Log in to your account and go to the page of the mission you wish to delete.
Then from open the hamburger menu at the top right of the screen.
Once opened, you can select the option to "Delete Mission"
Once selected, you will be prompted to confirm the deletion of your mission.
Note that mission deletion is permanent. If you delete a group mission, it will be deleted for everyone who is on that mission.
If you are on a group mission that was set up by someone else and you wish to be removed from the mission, you will need to make a request directly to the mission admin person for that mission.