While performing your workout routine, you are able to edit the values for the reps, sets and timer for each exercise within your routine.
Opening up JEFIT, you will want to click on the "Workout" tab on the top of the screen then select an exercise day that you want to perform for that day.
On the exercise list page to edit the reps/sets and timer for an exercise you will want to swipe to the left, revealing the "Edit" icon
From here you will release, when it is all the way to the left, in which the application will open up the "Edit Planned Values" page allowing you to edit the values for the reps, sets and timer for said exercise
You may also click on the "apply to all" button to set the values that you have entered in for ALL of the exercises within your planned exercises list.