Groups can't be deleted. You can:
Leave the group. Another member automatically becomes admin. See How to leave a group.
Remove all members and then leave, if you want the group to go quiet. See How to remove or block someone from your group.
Let a challenge end. It stops at its end date. You can shorten it by changing the end date in Settings → Basic Details. See How to edit group settings.
Switch it to a different type instead of starting over. See Change group types.
Want the same group again after a challenge ends? Use Restart. See How to restart a challenge.