To edit attributes in the Author mode, use the Attributes View.
The oxy_editor CSS extension function allows you to edit attribute and element text values directly in the Author mode using form-based controls. Various implementations are available out-of-the-box: combo boxes, checkboxes, text fields, pop-ups, buttons, which invoke custom Author actions or URL choosers. You can also implement custom editors for your specific needs.
As a working example, the bundled samples project contains a file called personal.xml, which allows you to edit attributes in-place using some of these default implementations.