The main page templates (FORUMHOME, SHOWTHREAD, FORUMDISPLAY, etc.) all have this in the template:
HTML Code:
{vb:raw header}
{vb:raw navbar}
So, you would need to swap those. However, you would also have to edit the two templates because the header starts with this:
HTML Code:
<div class="above_body"> <!-- closing tag is in template navbar -->
And the navbar has this line:
HTML Code:
</div><!-- closing div for above_body -->
So you would need to edit those two templates to correct those tags.
Alternately, you may want to try to play with the css. Add "top: -100px" to position the navbar and then add "top: 100px" to position the header. That may work out and be easier.