A workout can be added manually to the health app,firstly delete the incorrect workout;
Open the Apple Health app,tap on browse,then tap on activity,then tap on workouts,scroll to the bottom and tap show all data
Swipe left on the workout you want to delete and tap “Delete.”
The workout will be deleted, and the changes will sync back to your Apple Watch.
And in the fitness app tap on Workouts/Show More,then swipe left on the workout and tap delete.
Then;
In the Health App tap on the browse tab at the
bottom then tap activity and scroll down to workouts
then tap on workouts and in the the top right hand
corner of the screen it says add data,tap on that and
you can enter a new workout and when you're 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 as it cannot be added retrospectively.