Note: At the moment Lifesum doesn't support adding weight on one of the past days. You can only add a new weight today, or edit/delete a previous entry.
To edit/delete a previous entry tap on Progress tab at the bottom of the screen. After that tap on Body measurements and then on the button Update body measurements.
Then tap on Current under the Weight section. You will then see your weight history list. To edit a previous entry, tap on it, and then change the number. To delete a previous entry, tap on the cross next to the wrong value (right side of the screen). Please note that deleted entries are lost and cannot be restored.