There are two ways that you can delete Tasks from your To Do Lists. You can individually delete Tasks in your list or you can delete all of the finished Tasks in your list. Finished tasks are tasks that appear gray with a strike through the text to indicate that you have completed the Task.
Deleting individual Tasks
You can delete single Tasks in your list regardless of their status (Finished, unfinished, priority level, etc.).
Deleting finished Tasks
You can delete multiple tasks at once by click on the "All Tasks" button at the top of your To Do List and selecting the "Delete Finished Tasks" button. Follow the steps in the image below:
After you click on the "OK" button in the pop-up dialog your deleted tasks will be moved to the task trash can. These tasks will stay in the trash can for 30 days. After this point, the tasks will be automatically deleted. Once tasks are permanently deleted from your trash can there will not be a way to restore them.
Learn more about restoring deleted tasks from the trash can.
Learn more about how to delete entire To Do Lists.