I’m unsure what you mean by “middle” and “left”.
The theme’s header has a parent flexbox container (Row block) containing two child items (Site Title block and Navigation block). This effectively gives the header two main columns: the site title on the left and the navigation on the right.
So when you talk about “middle” and “left”, are you talking with respect to the overall header/page (ie 100% of the width)… or with respect to the Site Title on the left?
If you want to align the Navigation to the left of the Site Title, select the navigation block and change its justification from the default “Justify items right” to “Justify items left”. See screenshots below:
On the other hand, if you want to align the navigation to the left/middle of the entire width of the header, then you’ll first need to MOVE the navigation block above or below the Site Title block). That’s because you can’t align the navigation alone like this as long as there’s something else (the site title) taking some of the horizontal space.
Once the navigation occupies 100% of the header width all by itself, you can then, once again, change its justification from the default “Justify items left” to “Justify items right”. You may also have to set its “Alignment” to Wide or Full width as desired.