How can I change the power dictated in a workout after I have started the workout Garmin 830

I plan to do some indoor training (Wahoo Kickr) sessions using workouts I have saved on my Garmin 830.  My FTP is lower than it was when these workouts were created.  My question is, once  I start a workout, how can I change the resistance to make it either harder or easier?  Example, make the demand 90% of what is in the workout.  

I have seen examples of this on other Garmins, but not on my 830.  I can not figure out what buttons I need to hit!  Tx