Unfortunately, Apple Watch and the Health/Fitness app do not allow you to manually edit individual workout segment distances after the fact. Once a workout is recorded, the segment data is locked.
However, here are your options:
1. Edit the total workout: You can edit the total distance of a workout in the Health app on iPhone. Go to Health app > Browse > Activity > Workouts > tap the workout > Edit. You can adjust the total distance but not individual segment distances.
2. Use a third-party app: Apps like 'WorkOutDoors' or 'Strava' (if you sync workouts) may offer more editing flexibility for segments. If your workout synced to Strava, you can use their manual edit tools.
3. For future accuracy: On Apple Watch Series 5 with watchOS 10, for outdoor runs, try calibrating more precisely: Settings (on iPhone) > Privacy & Security > Location Services > System Services > Motion Calibration & Distance. Make sure this is enabled and do a few outdoor runs with GPS to recalibrate.
4. Treadmill calibration: For treadmill runs, you can improve accuracy by going to the Workout app on Apple Watch > Treadmill run > after the workout, it will give you an option to calibrate based on the actual distance.
Sorry there's no direct way to edit per-segment distances in the native app.