If i understand correctly, you need to set the margin between the page border and the top of the Header to a certain value. You can achieve this easily in LibreOffice Writer, so you don't need to manipulate the document in MS Word.
There are different spacing values you can set:
On the Page tab, you can set the Top/Bottom margins of the entire page. If you have a Header / Footer enabled, this is the margin between the physical "paper border" and the top border of the header) / the bottom border of the footer respectively. The Header / Footer is placed inside the margins defined on the page tab:
On the Header / Footer tab, you can set the height of the header / footer and the space between header / footer and main content. Indirectly, these values determine the margin of the main page content to the "physical" page border:
If there are certain, defined margins required, make sure not to choose Dynamic Space / Auto fit height on the Header / Footer tab.
So, the "big picture" looks like this (light yellow: main content; blue: header; green: footer):
d are set on the Page tab;
3a/3b on the Header / Footer tab respectively. The top margin of the "main content" area is the sum of
1c + 2a + 2b; its bottom margin vice versa
1d + 3a + 3b.