The main workout screen on the watch should look like this
It pulls the rep and weight from pre configured workout that you create like we do now.
then if nothing changed we just hit next and it cycles to whatever we have in our group or workout lineup. If we did some extra reps we up it and then hit next same if we change weights.
Now if someone is on our machine that we have in our next set we can swipe down to show this screen where we can see a list of the workouts in the routine. So we scroll down to the workout we want to do instead and hit a button to select then it goes back to the previous screen but for the new workout. Once we are done with that we can swipe up again and go back to the other workout once that machine is open.
Once the workout is complete Garmin remembers all the changes made for the next time you go to workout. So if originally my routine had bicep curl with 10 sets at 50lbs and this workout I did 10 sets and 60lbs then garmin will update my routine and next time it will show my new 10 sets and 60lbs.