If it is time to update your weight, the app will prompt you to do so.
Alternatively, you can click the blue-coloured plus button that appears on the bottom right corner of your screen then select Log Weight. This will take you to a screen where you can update your weight and click submit.
Another way of updating your weight is from the notifications you receive. Once a week, Fitwell will send a push notification asking you to update your weight. If you click on the notification from your lock screen or from within the app notification center, it will direct you to the weight update screen.