If you would like to change your workout plan completely, you can select the guide tab (dumbbell icon) to change to one of the other available guides.
If you would just like to update your existing workout plan, visit the home tab (home icon on the bottom left of the app screen) and find the week and date you want to amend from the top of the screen. From there you can select 'add workout' to add another workout session to your day.