Within Teamwork, you can change the creation and completion properties for completed tasks.
Note: Changing the completed date is only an option for site and project administrators.
Click the name of the completed task to open the task details view. In the left panel, hover over the Completed By section and click Modify.
In the modify modal, you can update the person who created the task, who completed the task, and the date the task was completed.
Click Apply Changes to update the task information.
For more information, see: Changing the Creator of a Task