The proper way is to change the page setup of the section to tell Word:
1. How far from the top of the page the header starts
2. How far from the top of the page the page content starts

There are plenty of ways to open the Page Setup dialog. One way is Layout > Margins > Custom Margins... (the lower option). The Margins tab controls item 2 above. The Layout tab controls item 1.

Note that if the header is too large for the space allotted to it, it overrides the item 2 setting to push the content further down the page. This is what jthomas666 was effectively telling you to do.

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Andrew Lockton
Chrysalis Design, Melbourne Australia