If you started the app without default weight unit, then picked the wrong one when configuring one, your old weights will be stored in the wrong weight unit. But don't worry, you can reset these changes. Here is how to do it:
1: Open the settings screen. On iOS this is located in the top right corner of the profile view. On Android it is located at the bottom of the main menu that you open by tapping the top left corner, when in the apps home screen.
2: Tap weight unit.
3: Tap the more icon in the top right corner.
4: Now tap reset.
That's it! Now you can reconfigure your weight units again.