Adding a header to a Microsoft Word document doesn't set the text in stone; you can go back and edit the header to amend or add information at any time in much the same way as the body text, headings, pictures or other content in the rest of the document. By default, the header text or image appears on every page of the document, so adding another line of text to an existing header will change the header throughout the document. Add a line to your document's header with the Edit Header tool in Microsoft Word 2013.
Double-click anywhere inside your document's existing header area to launch Word's header and footer tools. Alternatively, click the "Insert" tab, select "Header" in the Header and Footer tools group and then click "Edit Header" in the drop-down menu.
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Click the page at the end of the existing header line and then press the "Enter" or "Return" key to insert a line break. Type the additional header line. Word propagates the change across the entire document, adding the extra line to the header section on every page.
Adjust the distance between the top of the document and the header. Enter or select with the arrow button your preferred distance in the "Header from Top" input field that you'll find in the Position tools under the Design tab. Adding extra space above the header is sometimes useful in large printed documents where a staple or fastener might obscure the header text as the reader turns pages.
Highlight the header text if you want to change the font face or style the text. Select your preferred font face, size, colors and styles using the Font tools under the Home tab. Using bold or underlined text is useful if the header also serves as a document title or subtitle that needs to stand out.
Click the "Design" tab on the main menu ribbon and then select the "Close Header and Footer" button to close the tools and return to the main document. Alternatively, double-click anywhere on the main body of the document to exit the tools.
Save the document to store all changes.