Here's how to swap out an exercise in the "Day View" of the Workout Plan Creator.

Step 1: Click the "swap" link for the exercise you want to replace.

 

Step 2: Use the search menu to select and confirm the new exercise you want to add to the workout.

 

NOTE: If you are swapping to a custom exercise that has default notes entered, the default notes will overwrite any notes you had entered before swapping the exercise.