record weight loss
VSG on 10/16/12
VSG on 02/18/13
Go to your profile and click "Health Tracker" by your picture. On the right hand side of the page a link will say "edit my health tracker". That is where you enter your new weight. Just remove the old weight and enter the current. It will update your ticker.
*edit* There may be an easier way, but that's the way I have found so far.