Hover over the task box until the appears, then click on it to mark it as complete. If you complete a shared task, you will have an option to either to mark this task as complete on your list or on all members’ lists. If you choose ‘Everyone’, all the members will receive a pop up notification and the task will be removed from their lists. If you choose 'Just mine', it will be completed on your list and you will be removed from the shared task. The other members will receive a notification that you have left the task in their Notification Center.