Most components will have editable attributes, such as the resistance for resistors. By default, you can edit simple attributes in the toolbar, when a group of the same type of components are selected. If your selection contains a mixture of different types of components (such as resistors and capacitors), then no attributes will be displayed for editing.
Some components have more advanced attributes which are not accessible via the toolbar. These are found in the Item sidebar on the right. The diode, for example, has a variety of behavioral characteristics that you can edit here.
There is one type of attribute that cannot be editable by either the toolbar or Item sidebar - multiline text. Double clicking on the item will bring up a dialog box where the text can be entered.