Test session due dates can be edited until the test session is marked as final.
Navigate to the Test Coverage primary tab, and then to the Builds secondary tab. In the specific build's details window, click the Test Sessions tab to view the test sessions for the build. Select the test session to view the details window. Click the icon under the Due Date field (or the current due date if one had been previously set) to select the desired date using the calendar pop-up.
Users will need view and edit permissions in the test coverage module to edit the due date. Depending on the settings of a users project permissions, users may be able to view all project builds or just a subset of them. Project owners manage the permissions for project members.