I think my scales might be off. How would I know. When i get on the scale it first appear with -- on the scales. My sister said that she went and got weigh in and the scale told her it was like 8 lbs in 2 days she lost. She went to Weight Watches and the scales there had her as add 1 lbs compare to what she weigh before she notice the scale was off. How can I reset the scales to be correct?