Note: Once you delete the start weight entry you won't be able to edit it or change the start weight. Then the next weight entry will automatically become your start weight.

It is not possible to change the date for a weight entry, just the weight on a specific date.
 
  • Go to Progress -> Body Measurements.
  • Tap on the green button Track body measurements.
  • Tap on Current in the weight section. In this view you can see the history of your weight. Scroll down till you see the first tracked weight - your start weight.
  • Tap on the number and change it to the desired amount.
  • Confirm by tapping on green button 'Save'.
  • Check your weight tracker: your start weight is now updated.
 
Alternatively you can set a completely new start weight by settings a new goal.