It depends on the field type. If this is a field of type number or text, you can simply click into that field on a layout and edit it. A script can use Set Field to modify it also.
If this is a field of type calculation, then you cannot directly modify the field. You can edit any number or text field that it references in its calculation expression or you can open up Define Fields and modify the field's calculation definition. (And checking the field's definition is the best way to determine it's field type.)