How to display completed agenda items?

After adding some task items to the agenda list, I clicked the check box to mark as complete. The items then disappeared. How do I get them back displayed on the agenda list as completed?

Unfortunately that is not possible. Once a task has been marked as complete, it will not display in Agenda, even it it has a future due date.

Is it possible to undo the completion state and return the item to the list as it originally was?
Or is it possible to locate the entire  list of agenda items and edit them?

If you click on the tasks tab, you can choose to display or not display completed tasks

Yes you can change the status, and as long as the due date is not earlier than yesterday, it will again show in the Agenda.

Go to the Tasks Tab. If you don’t see the completed tasks, click on the top menu bar, and disable Hide completed tasks. Then you can change the status of any you marked as complete.