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"
To add an exercise
A: Click Add Practice. You will then get up choices for how exercise you want to add.
B: Create your own exercise now
C: Select one of the exercises from the library
D: Remember to save your changes at last.