You can reschedule a workout plan that your client is following via the "Manage" link on the "Clients" tab of the Trainer Dashboard:
From the Manage Client view, click "Workouts" located on the left side of the page.
Click the "Manaage" link to make changes to your client's current workout schedule:
Adjust your client's schedule and 1RM info as needed, then make sure that you choose the correct workout under "Choose the first workout.", based on your client's current progress in the workout plan.
Click the "Update Schedule" button, and your client will now have an updated workout schedule, starting from the workout that you chose!