With watchOS 6 I had problems too. Here is how to change your move goal:
- Open the Activity app on your Apple Watch.
- Go to your rings, then firmly press the screen.
- Then tap "Change Move Goal".
- Tap to increase or decrease the number of active calories for your daily Move goal.
- When you're done, tap Update.
For some reason (with watchOS 5 and 6), you can change the Move goal, but not your Exercise or Stand goals. This was changed in WatchOS 7 so that you can now change all the goals. See the "Change your activity goals" section in:
Use the Activity app on your Apple Watch - Apple Support