Editing a pdf in Preview - how to type in a text box inserted into the pdf

You double-click on the boilerplate "Text" that is inside the Text box and commence typing your own text. You may wish to expand the Text box laterally before typing, and text will automatically wrap when the width of the text box is exceeded.

As long as you don't do anything but type after entering the text box, you will edit the text.

Otherwise, you have to select it again and highlight the text.