apple watch workout

Go to Apple Health app on the phone tap on Browse at the bottom then tap Activity then tap on Exercise Minutes scroll to the bottom and select Show all data then tap on the date that you want to remove then swipe left on the day in question and tap on delete.,when you're finished tap on done in the top right corner.

You can delete the workout an then add a new one manually but it won't change the 3 activity rings.

You can delete exercise minute by minute and that will change the old number,it only works for the present day though.