How do I change the difficulty of workouts?

You can change the difficulty level of your workouts in the My program tab. 

IOS:

Press 3 horizontal lines in the upper right corner:

Press Difficulty and Choose:  

After that, change your program by pressing Save:

Android:

Go to the My program tab. Press 3 horizontal lines in the upper right corner, then select My program: 

Press Difficulty and adjust as needed. Then press Update difficulty.