How can I restore an exercise I accidentally deleted from the Workout app?

To manually add a workout (and its data):

1. In the Health App on the iPhone, tap on the Health Data tab at the bottom
2. Then tap Activity and scroll down to Workouts and tap on it
3. In the the top right hand corner of the screen it says add data (or simply +)
4. Tap on that and you can enter a new workout and its data
5. When you are finished tap on add at the top right of the screen.

Be aware though that this only effects your 3 rings on the day in question. And it may not work with the latest watchOS versions.