How to interrupt a global header in Microsoft Word
To interrupt a global header, go to the first page of the section after the section that should not have this header, enter the header pane by double clicking at the top side of the page, go to Header and Footer Tools at the ribbon, and press the Link to Previous button to remove the link.
The button is deactivated and this header won’t follow anymore what is written in the header of the previous section. Then go to the section you want to customize; unlink this too from its previous, and now it’s safe to edit your header without changing anything in other sections’ headers.
Repeat the same steps three times, for the first, the second (even) and the third (odd) page of all sections you unlink, in case you configured (at the Page Setup of your document) your sections to have different odd and even headers and footers and different first page.
If more sections in your document need a new singular header, follow the same routine, unlink their headers from the previous and next section’s headers.
Since a header inherits all content from the previous section’s header, if you want to interrupt this continuation in several parts of your document, start from the very last, change the last section that needs a custom header, then go on to change the preceding section’s header that needs to be changed, etc. This helps to avoid mistakes and unnecessary editing, since sometimes we unlink the current section from its previous, but we forget to unlink also the next section from the current, this way applying without knowing it our edits to each and every section after the current.