The day before yesterday: went swimming but watch did not auto record so manually entered data (time, distance etc) after exercise.
Yesterday: went swimming - watch auto-recorded time and heart rate but not distance. There doesn't seem to be a way to enter this additional data, nor does there seem to be a way to delete the auto activity and add a new one for yesterday?
It's not possible currently to update an existing workout in Samsung Health. You could however use another app that can sync data with Samsung Health and add the activity there or use it to record activities.
You could also submit a feature request via the Support --> Send feedback --> Ask a question ( I did it couple of weeks ago) via the Samsung Members app or through Samsung Health --> Settings --> Contact us --> Ask a question. The more people ask for this feature, the more likely it is that they will consider to include it in one of their updates.