easiest way to move undone items to today
in the 7 days view, you can simply drag & drop tasks that are overdue and put them to the "Today" section of your task list (to drag & drop a task, use the handle on the left side that appears if you hover over a task).
Alternatively you can use the multi-select option. To do so, press and hold the Ctrl key and start selecting tasks by clicking on them. Once you select them, you can choose "change date" or "postpone" from the menu at the top, but please note that "Change date" will change the date formula of a certain task and this cannot be performed for recurring tasks and the "Postpone" option will move the date to the task's next occurrence (not "today").