You may encounter tables of data throughout the lessons, as well as be asked to fill in answers in these tables. You'll know a cell is editable if it has a grey block in it (for example, in the "12" column below). Some table headers can also be edited. You'll know that the header is editable if a blue "edit" link is displayed.
To edit a cell or header, simply click on it and begin typing. Your answer will be saved as soon as you click outside the box.