This can (currently) only be done on your computer.
Open Training Plans and find the training plan you want to edit/change.
#1 Click the menu icon on the right side of the training plan.
#2 Select "Edit"
You are now free to make changes to your training plan.
A: Change the minutes the exercise should be performed
B: Select exercise from the entire practice library
C: Sort the order of exercises with arrow keys
D: Delete the exercise from the training schedule. The exercise will still be found in the library.
E: Remember to save your changes at last.