If you want to adjust when your program start date, follow these steps:
Open the On-demand workout library
Tap Explore from the bottom left corner of your screen. Make sure Workouts is selected from the top menu.
Select your active program
This will be the program with Active tagged in the top left corner.
Update your schedule
Hit the Update schedule at the bottom of the screen.
Choose your new start date
Select the date you want to begin and tap Add to Schedule.
Your workout program will now be updated with the new start date.