You should choose an appropriate view before beginning this operation. Only the To-do List view and the To-do View sidebar component show to-dos which do not have a due date.

To display the To-do List view, press the To-do List view toolbar button, or choose the View → To-do List menu item.

To reschedule a to-do, right click it. This will bring up a context menu. Choose the Copy to or Move to menu items and select the new date from the calendar now appearing. The to-do is now copied or moved to the selected date. Alternatively, if the main panel holds a To-do List view, just right click the column Due Date/Time. A calendar will appear instantly, allowing you to set the new due date.

To edit a to-do, right click it and choose the Edit... menu item.