Microsoft Word 2016, 2019, Microsoft 365, or Word Online:
Open your Word document.
Navigate to the top of the first page where you want to add the header.
Double-click on the top margin of the page. This will open the Header & Footer Tools Design tab.
In the "Options" group on the Design tab, check the box that says "Different First Page."
Once you've selected "Different First Page," you can add your header for the first page. This header will only appear on the first page of the document.
To go back to the body of your document, click the "Close Header and Footer" button, or simply double-click in the body of the document.
Now, the header on the first page will be different from the rest of the document.
Microsoft Word 2013:
The process is similar to the newer versions, but the interface might be slightly different:
Open your Word document.
Go to the top of the first page where you want to add the header.
Double-click on the top margin of the page to open the Header & Footer Tools Design tab.
In the "Options" group on the Design tab, check the box that says "Different First Page."
Add your header for the first page.
To return to the body of your document, click the "Close Header and Footer" button, or double-click in the body of the document.
Microsoft Word 2010 and earlier:
Open your Word document.
Go to the top of the first page where you want to add the header.
Double-click on the top margin of the page to open the header.
On the toolbar, select the "Page Layout" tab.
In the "Page Setup" group, click on the "Different First Page" checkbox.
Add your header for the first page.
To return to the body of your document, double-click in the body of the document.
Remember to save your document after making changes. This way, the different first-page header will be retained when you close and reopen the document.