To shift the dates of a training plan that you have already applied to your calendar, click on the main Training Plans link at the top of the log. Then, click the "Plan History" button on the right to be taken to your history page here: https://log.finalsurge.com/TrainingPlansHist.cshtml
You can then click the pencil edit icon beside the plan that you want to shift. Shifting a plan will only shift workouts that you have not already completed.
Below is a video which shows how to do this: