How can I change the number of workouts per week?

You can easily change the number of your workout days per week in the My program tab. 

Press 3 horizontal lines in the upper right corner, then select Workout schedule: 

Here, you can see your current workout days. To adjust your schedule, uncheck the days you don’t want to work out and tap on the days you’d like to have workouts. 

Click Set Up and your workout schedule will be updated.