You should be able to go back to the day where the measurement was recorded and edit it on your weight/measurement page. I'm not sure if that's different if you're using the Withings scale synced to SparkPeople.
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford
"No matter how slow you go, you are still lapping everybody on the couch."