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 ME tab at the bottom of the screen (icon showing a profile). After that tap on the Body icon and then on the button Track body measurements.

Then tap on the tab Current under 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 cross next to the wrong value (right side of the screen). Please note that deleted entries are lost and cannot be restored.