A family of Microsoft word processing software products for creating web, email, and print documents.
Press Alt+F9 (Fn+Alt+F9 on some computers) to display field codes. If the text in the heading is a StyleRef field, it will reflect the text in the document that is formatted with that field. You will need to make sure that there is a paragraph in that style that contains the appropriate text.
It's also possible that there is a content control in the document that asks for the document title, which could then be saved as a Title document property, so the text in the header might be a Title field. Check the document properties to make sure the Title field is correct.