To edit a field you should first select it by double-clicking in the field. The Field menu option is displayed along the top of the window. Clicking on any of the options in the Field menu brings up the "Field Attributes" dialog. In here you can change or add to attributes already set. The following options are available:
General contains details of the field type - either formula or database - and field name. You can change these here.
Color allows you to select a color for the field.
Font enables you to choose a font and special effects for text within the field.
Border contains various border types for the field.
There are also options for Alignment, Style, Control, Attributes, and Format.