Rep counting

What type of change? Some changes can be made on the watch - Venu 3 series Owner's Manual - Recording a Strength Training Activity (garmin.com)

Or maybe you're referring to something like "skip group," which allows you to skip an exercise and go to the next one, come back to it later. Often done if you're at a public gym and the machine is in use.

Skipping a Group
If you have started a workout and need to skip a step in a workout, you can skip the group and come back to it later. Not all Garmin watches support this feature.

Thanks for replying. The method referred to in the owners manual does not apply to Custom Workouts. On leg day for example, I’ll start my workout and the first exercise is listed. The rep counter will start, keep track as best it can if there is a discrepancy, and when I complete the set, I am not sure how to edit for rep number or weight used. I know how to skip sets using the top button, but pressing the lower button just takes you to the next exercise.  The edit works fine for an impromptu workout as described by the manual.