Is there a way to change the meter length after a swim workout?

Do you mean edit the distance swam?.

If so you can delete the workout and enter a new one in the health app.

Are you talking about the length of the pool?

For Pool Swim, turn the Digital Crown to set the pool length. This helps your Apple Watch accurately measure your laps and distance. Then tap Start.

1. Open the Workout app. 
2. Scroll to Pool Swim or Open Water Swim. Use Pool Swim for laps in a pool and Open Water Swim for swimming in places like a lake or ocean.
3. Tap to start the workout or tap the more button  to set a calorie, distance or time goal.
4. For Pool Swim, turn the Digital Crown to set the pool length. This helps your Apple Watch accurately measure your laps and distance. Then tap Start.
5. Wait for the three-second countdown. To skip the countdown, tap the screen.